What can be determined by the average number of people per one unit of area?
Options:
- (A) hygeine
- (B) emissions
- (C) animals
- (D) growth
- (E) forces
- (F) health
- (G) education
- (H) people
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Let's think first: Population density is the average number of individuals per unit of area or volume. Another measure of growth is population density. Growth of an area can be determined by the average number of people per unit of area.... So the answer is [(D)]

Scientists study animal behavior through which of the following?
Options:
- (A) audio and video tape
- (B) Electrical energy
- (C) irradiation
- (D) A computer
- (E) raising their temperature
- (F) Something coming from a gland
- (G) The bending of a ray of light
- (H) characteristics
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Let's think first: Ethologists usually study how animals behave in their natural environment. Many ethologists capture their observations on videotape or audio tape. Scientists who study how animals behave use videotape or audio tape.... So the answer is [(A)]

What are critical for erectile health?
Options:
- (A) fur and fat
- (B) Energy.
- (C) Energy
- (D) sweating
- (E) exercise
- (F) arteries
- (G) strength
- (H) a cuticle
--
Let's think first: Arteries generally carry oxygen-rich blood. Oxygen-rich blood is critical for erectile health. Arteries are critical for erectile health.... So the answer is [(F)]

What obtain their food from plants and cannot make their own food?
Options:
- (A) bacteria
- (B) Flowers
- (C) trees
- (D) lichen
- (E) animals
- (F) fungi
- (G) humans
- (H) grass
--
Let's think first: Heterotrophs are living things that cannot make their own food. All animals are heterotrophic and obtain their food from plants. Animals obtain their food from plants and cannot make their own food.... So the answer is [(E)]

_ contribute to soils.
Options:
- (A) Sediment
- (B) Aphids
- (C) cacti
- (D) Roundworms
- (E) Exfoliation
- (F) Amphibians
- (G) Ringworms
- (H) rainfall
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Let's think first: Roundworms make up the phylum Nematoda. Also, nematoda and rotifera are important groups contributing to soils. Roundworms contribute to soils.... So the answer is [(D)]

What can polar bears do to generate heat?
Options:
- (A) make fire
- (B) eat ice
- (C) stay warm
- (D) nothing
- (E) fur
- (F) motors
- (G) shiver
- (H) sweating
--
Let's think first: Mammals can also generate little bursts of heat by shivering. Polar bears are mammals. Polar bears can generate heat by shivering.... So the answer is [(G)]