a week ago 一周前,用于一般過去時.
since last week自從上周以來,用于現(xiàn)在完成時
He came here a week ago .
He has been here since last week .有個問題是甘的我不是很明點解選b hao long did you get ill?-----------------a a week b a week agoc since last week應(yīng)該意思上c才對的問句是一般過去時，回答也應(yīng)該用一般過去時，故B對。