Question

______ you've been gone I only cook ______ myself. A) Since/ for B) Now/ for C) Since / and D) For/ since E) Already/ since

Answer 1

Thank you for asking the question!
Since you have been gone I only cook for myself. 
In this sentence since is selected because the specific time is not mentioned. So when the exact tie span is not given then since is used. There is very thin line between the usage of since and for so it must be kept in mind. 
Hope it helps.