The answer is b but depending on your situation it could b c a computer is always multitasking in the background but when c happens usually too many things are writing to your disk at once and you need to switch a portion of your available disk space to that new program, a usually never happens unless you have a really bad computer and b is what usually happens when using a computer so again b is the answer but be careful when making that decision on a test if it gives you a scenario check the disk usage.
Hope this helped
