题目

若Sn为数列{an}的前n项和,且Sn=,则等于( ) (A)    (B)    (C)   (D)30 答案：D解析:a5=S5-S4=-=, ∴=30.Mobile phones have made our lives much _______ than before.A.easy B.easiest C.the easiest D.easier