Hi Tracyyahoo,

Try to avoid use of Perfect tense unless we don't have any other option as correct answer.
For sake of understanding,
We use present perfect tense for two purpose
1. To show that any action is continuous from past and still in active status or if action is over at least its effect is there.
2. To show the sequence of events.
Here whatever is coming after "heat-exchange network" is describing/modifying only "heat-exchange network ". Now if we will use perfect tense then the sentence will be like below.
heat-exchange network which has kept the brain from getting too hot.
So here we don't have sequence of events, no action which should have completed till now or any effect" so use is wrong.

Anyway for regular thing we use simple tense, that is option B.

KISS = "Keep it simple Sir"

I hope my explanation helped, If indeed please give me kudos.
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Thanks Rphardu

Can someone please explain what is wrong with (C) and D?

A few things to watch out for:

Which vs. That

The sentence is describing a special 'heat-exchange network' not all 'heat-exchange networks.' In the latter case, we would use 'which.' A heat-exchange network, which is found in both nature and computing, has many practical advantages. However, in this sentence we are not describing all 'heat-exchange networks' only the one relating to carnivorous animals. Thus, we use 'that.'

Another tip off: there is no comma between 'which' and the noun it is modifying. Used in this manner 'which' must always be separated from the subject it is modifying by a comma. 'That', on other hand, does not take a comma.

Present Tense vs. Perfect Tense

The present tense is used to describe an unchanging characteristic of something:

1) Running several miles each day burns many calories vs. Running several miles each day is burning many calories.

2) The sloth is a mammal known to move with more deliberation than the most circumspect of snails vs. The sloth has been a mammal known to move with more deliberation than the most circumspect of snails.


Carnivorous mammals can endure what would otherwise be lethal levels of body heat because they have a heat-exchange network which kept the brain from getting too hot.
a) which kept
b) that keeps
c) which has kept
d) that has been keeping
e) having kept

Hope that helps
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Christopher Lele
Magoosh Test Prep

To my understanding, The basic difference b/w usage of "Which and that" is ,
Which is used with non essential Modifiers, for instance, This Car, which is painted red, is mine,

where as

That is used with a essential modifiers, for instance, The car that is painted red is mine.

Here in the question usage of That is justifiable based on the above mention rule.

Thanks