I've got two more questions, please.
1- What does the sentence with (b) as an answer mean?
Here is the sentence with answer (b):
(b) There are times when you can't work hard enough to live comfortably.
That sentence means that there are times when you can't afford to live comfortably no matter how hard you work.
2- If 'too' is preceded with a negative verb, I get confused and can't understand meaning of the sentence clearly. Is there a way to get past that? The problem arises from the fact that I automatically add a negative meaning to verb following the 'to'. In other worsd, when we read "I am too short to reach the shelf" I interpret the sentence as follows "I am very short. I can't reach the shelf."
Well, let's add sentential negation to "I am too short to reach the shelf":
(X) I am not too short to reach the shelf.
That sentence means you are short but you can nevertheless reach the shelf.