The phrase "all still in high school" is an interrupting phrase, one that provides extra information to the sentence but is not a part of the sentence's main structure. Because of this separation, all interrupting phrase should be surrounded by commas; therefore, the correct answer choice is "The debaters, all still in high school, showed a maturity beyond their years."

When additional elements are put after or before the subject (in this case, "unaware of their country's") it should be separated by commas. Also look for the answer where "country" is in its proper possessive state: "their country's tense foreign relationship."

The phrase "a long time civic activist" is an appositive, a noun that renames a previously introduced noun while providing more information about that previously introduced noun. Appositives that can be removed from a sentence without affecting the sentence's meaning must be set apart by commas. "The speaker, a long time civic activist, urged" is the only answer choice that actually does this.

The phrase "all rescued from local shelters" is an interrupting phrase, a phrase that gives additional information to the sentence but remains apart from its structure. All interrupting phrases must be set apart from the rest of the sentence by commas. The only answer choice that appropriately does this is "The dogs, all rescued from local shelters, took quite a while to adjust to their new foster homes."

The phrase "held every four years" is an adjectival participle phrase, one that sits outside the structure of the sentence, but provides extra information. Any such phrase must be set apart by commas. "The tournament, held every four years, is" is the only answer choice that appropriately does this.

The phrase "originally scheduled for three hours" is an interrupting phrase, or one that sits outside the sentence's structure but provides extra information. All interrupting phrases must be set apart by commas. The only answer choice that appropriately does this is "The dance, originally scheduled for three hours, had to be cut short after just an hour.

The phrase "said to be slimming" is an interrupting phrase, a phrase that provides extra information to a sentence, but is not a crucial part of its structure. Any interrupting phrase must be set off from the rest of the sentence by commas. Therefore, the correct answer choice is "Black clothing, said to be slimming, actually depends on the wearer’s shape for its slimming qualities."

The phrase "which broke new ground in the medium" is an interrupting phrase, a phrase that provides information but is not crucial to the structure of the sentence. All interrupting phrases must be set apart from the rest of the sentence by commas. The only answer choice that appropriately does this is "The advertisement, which broke new ground in the medium, remained."

The phrase "a familiar presence on local radio" is an interrupting phrase, one that provides extra information to the sentence but is not part of the sentence's necessary structure. All interrupting phrases must be set apart from the rest of the sentence by commas. The only answer choice that correctly sets off the interrupting phrase is "The sportscaster, a familiar presence on local radio, announced his retirement earlier this month, at the age of eighty-seven."

The phrase "Moving away from his hometown" is an introductory phrase, one that conditions the main action of the sentence while remaining separate from the sentence's main body. All introductory phrases must be set apart from the rest of the sentence by a comma. "Moving away from his hometowm, the college student wanted" is the only answer choice that appropriately does this.