I'll give you the most efficient way to get through these questions, in my opinion, since you haven't asked for specific explanations on answer choices -- if you need more specific help, though, certainly let us know.
In this question, "for" actually is used to mean "because."
In order for the meaning to make sense, we need to say that the cello performance saved the show BECAUSE the rest of the show was disappointing. This is the quickest way to solve this problem, and this method rules out B, C, D, and E as answer choices. All of those have a meaning that doesn't fit with the author's apparent intended meaning.
The simplest method on this one is to look at the structure of the sentence that involves
"...animals ... either migrate... or try to survive..."
Since 'migrate' and 'try' need to match, we're left only with answer choice D as an option.
To modify non-countable things like water, we have to use the term "little" and to modify countable things like trees, we must use the term "few" instead.
We can not use either 'little' or 'few' to modify both 'water' and 'trees.'
A uses 'little' to modify both terms, as does B.
C uses the wrong verb form -- it should be plural (exist) to match 'trees.. and water'
D uses 'little' to modify both terms.
E uses 'few trees' and 'little ... water' correctly, and uses a plural form of the verb, so is our correct answer.
If you need further elaboration on these, let us know.