Use of 'which'

Thanks BKimball.

I was never aware of this comma rule as I have never came across an answer choice that was wrong simply because it was missing a comma before 'which' when used as a noun modifier.

Hi expert,

I have a question about "which". I understand that when used as a modifier, it must "touch" the noun it modifies and is usually used as a modifier that doesn't narrow the topic of the noun.
Now, I have 2 questions that I don't understand:
1. We become more alert and responsive, which is appropriate when we’re in danger. This is a sentence from a highly respected review. Is the use of "which" here valid (for GMAT purposes)?
2. I encountered this GMAT question:

Gusty westerly winds will continue to usher in a seasonably cool air mass into the region, as a broad area of high pressure will build and bring fair and dry weather for several days.

A. to usher in a seasonably cool air mass into the region, as a broad area of high pressure will build and
B. ushering in a seasonably cool air mass into the region and a broad area of high pressure will build that
C. to usher in a seasonably cool air mass to the region, a broad area of high pressure building, and
D. ushering a seasonably cool air mass in the region, with a broad area of high pressure building and
E. to usher a seasonably cool air mass into the region while a broad area of high pressure builds, which will

Here the right answer is E, even though the verb builds separates between "which" and the noun it modifies "a broad area of high pressure"!
How am I supposed to know when a verb can make this separation? I've heard about a theory that "builds" act as a linking verb (since when?), so it doesn't really separate the two.

Please help

Mike