a week ago 一周前,用于一般过去时.
since last week自从上周以来,用于现在完成时
He came here a week ago .
He has been here since last week .有个问题是甘的我不是很明点解选b hao long did you get ill?-----------------a a week b a week agoc since last week应该意思上c才对的问句是一般过去时，回答也应该用一般过去时，故B对。