Adapted from A Catechism of Familiar Things: Their History and the Events Which Led to Their Discovery by the Benziger Brothers (1881)

Twilight is the light from the first dawning of day to the rising of the sun; and again between its setting and the last remains of day. Without twilight, the sun's light would appear at its rising, and disappear at its setting, instantly; and we should experience a sudden transition from the brightest sunshine to the most complete obscurity. The duration of twilight is different in different climates; and in the same places it varies at different periods of the year.

This passage is primarily concerned with __________.

Adapted from "Some Strange Nurseries" by Grant Allen in A Book of Natural History (1902, ed. David Starr Jordan)

Many different types of animals employ one of two strategies in raising their young. Certain animals, called “r-strategists,” turn out thousands of eggs with reckless profusion, but they let them look after themselves, or be devoured by enemies, as chance will have it. Other animals, called “K-strategists,” take greater pain in the rearing and upbringing of the young. Large broods indicate an “r” life strategy; small broods imply a “K” life strategy and more care in the nurture and education of the offspring. R-strategists produce eggs wholesale, on the off chance that some two or three among them may perhaps survive an infant mortality of ninety-nine per cent, so as to replace their parents. K-strategists produce half a dozen young, or less, but bring a large proportion of these on an average up to years of discretion.

Which of these extracts best captures the main idea of this passage?

Adapted from "Rain and Snow" by John Tyndall in Wonders of Earth, Sea, and Sky (1902, ed. by Edward Singleton Holden)

At the equator, and within certain limits north and south of it, the sun at certain periods of the year is directly overhead at noon. These limits are called the Tropics of Cancer and of Capricorn. Upon the belt comprised between these two circles, the sun's rays fall with their mightiest power, for here they shoot directly downwards and heat both earth and sea more than when they strike slantingly. When the vertical sunbeams strike the land, they heat it, and the air in contact with the hot soil becomes heated in turn. But when heated, the air expands, and when it expands, it becomes lighter. This lighter air rises through the heavier air overhead.

When the sunbeams fall upon the sea, the water is warmed, though not so much as the land. The warmed water expands, becomes thereby lighter, and therefore continues to float upon the top. This upper layer of water warms to some extent the air in contact with it, but it also sends up a quantity of aqueous vapor, which being far lighter than air helps the latter to rise. Thus both from the land and from the sea we have ascending currents established by the action of the sun.

The primary function of this passage is __________.