How are atolls built?
Options:
- (A) important habitats
- (B) By coral exoskeletons
- (C) by fish
- (D) with machines
- (E) by a type of eel
- (F) It used to be covered by water
- (G) The area was once underwater
- (H) reusing materials
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Let's think first: Many corals secrete an exoskeleton that builds up to become a coral reef. An atoll is the third type of reef. Atolls are built by corals secreting an exoskeleton.... So the answer is [(B)]

There are what who study how animals behave in their natural environment.
Options:
- (A) infants
- (B) heat or cold
- (C) insects
- (D) sensory neurons
- (E) scientists
- (F) people
- (G) humans
- (H) reptiles
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Let's think first: Ethologists usually study how animals behave in their natural environment. Ethologists are scientists who study animals in their natural habitats. There are scientists who study how animals behave in their natural environment.... So the answer is [(E)]

All redwoods have a characteristic life cycle that includes what?
Options:
- (A) seed dispersal
- (B) alternation stations
- (C) biological diversity
- (D) The flowers are pollinated
- (E) sub-units of species
- (F) alternation of generations
- (G) alternation alteration
- (H) alternation of genetics
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Let's think first: All plants have a characteristic life cycle that includes alternation of generations. All trees are plants, including redwoods. All redwoods have a characteristic life cycle that includes alternation of generations.... So the answer is [(F)]

Trees have woody stems covered with what?
Options:
- (A) heat energy
- (B) thin skin
- (C) protective skin
- (D) skinny cells
- (E) white skin
- (F) Chlorophyll
- (G) Organic compounds
- (H) looseness
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Let's think first: Trees have woody stems covered with bark. On the outside is a protective skin of bark made mainly of cork. Trees have woody stems covered with protective skin.... So the answer is [(C)]

What do constructions by earthworms help to do?
Options:
- (A) compact soil
- (B) moisten soil
- (C) agriculture
- (D) to move people
- (E) relocation
- (F) aerate soil
- (G) survival
- (H) dry out soil
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Let's think first: Earthworm burrows help aerate soil, which is also good for plants. Most species construct burrows. Constructions by earthworms help aerate soil which is good for plants.... So the answer is [(F)]

What does the human immunodeficiency virus destroy?
Options:
- (A) B cells
- (B) bacteria
- (C) helper T cells
- (D) animals
- (E) epidermis and dermis
- (F) critical organs
- (G) an organism's body
- (H) cancer cells
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Let's think first: HIV infects and destroys helper T cells. HIV, the human immunodeficiency virus, is a retrovirus. The human immunodeficiency virus infects and detroys helper T cells.... So the answer is [(C)]