Meaning: 'Living' refers to the state of being alive or existing, indicating that these animals are currently active in their environment.

Wrong options:

• lived: The word 'lived' is the past tense and does not fit the present context of the sentence, which discusses current behaviors of animals.
• being lived: 'Being lived' is grammatically incorrect in this context as it suggests a passive construction that does not apply to animals actively existing in the desert.
• have lived: 'Have lived' implies a completed action in the past and does not convey the ongoing state of existence required in this sentence.

Correct answer: less active

Explanation: The phrase 'less active' appropriately describes the jackrabbit's behavior at night, indicating a comparative state of activity that fits grammatically and contextually with the sentence's structure.

Meaning: 'Less active' means not as much energy or movement compared to another time, in this case, the daytime when the jackrabbit is more active.

Wrong options:

• actively: 'Actively' is an adverb that does not fit the context as it does not describe the state of being but rather the manner of action, which is not what is needed here.
• more actively: 'More actively' suggests an increase in activity, which contradicts the context of the jackrabbit becoming less active at night.
• active: 'Active' is an adjective that does not convey the comparative aspect needed in this context, as it does not indicate a change in activity level.

Correct answer: off

Explanation: The word 'off' correctly indicates the action of giving away heat, fitting grammatically and contextually as it describes the function of the jackrabbit's ears in relation to temperature regulation.

Meaning: 'Off' in this context means to release or emit, referring to the jackrabbit's ears dispersing heat to cool down.

Wrong options:

• up: 'Up' does not fit the context as it implies direction rather than the action of releasing heat, making it grammatically incorrect here.
• over: 'Over' suggests a covering or excess, which does not align with the intended meaning of heat emission.
• in: 'In' implies containment, which contradicts the action of giving off heat, making it unsuitable for this context.

Correct answer: if

Explanation: The word 'if' introduces a conditional clause, fitting grammatically and logically as it sets up a scenario where the jackrabbit's action depends on the presence of danger.

Meaning: 'If' indicates a condition that must be met for the action to occur, in this case, running when danger threatens.

Wrong options:

• unless: 'Unless' introduces a negative condition, which does not fit the context of the jackrabbit running when danger is present.
• until: 'Until' suggests a time limit rather than a condition, making it grammatically incorrect in this context.
• suppose: 'Suppose' implies a hypothetical situation but does not function as a conditional conjunction, making it unsuitable here.

Correct answer: up to

Explanation: The phrase 'up to' correctly indicates a maximum limit of speed, fitting grammatically and contextually as it describes the jackrabbit's potential speed in a clear manner.

Meaning: 'Up to' means a maximum amount or limit, indicating that the jackrabbit can reach speeds as high as 56 km/h.

Wrong options:

• as far as: 'As far as' typically refers to distance rather than speed, making it contextually inappropriate here.
• until: 'Until' suggests a time frame rather than a limit on speed, which does not fit the context of measuring speed.
• faster than: 'Faster than' implies a comparison to another speed, which is not the intended meaning in this context.