a recent series of rape cases involving teenagers __________ grave enough to attract our special concern.
(a) has (b) have (c) has been (d) have been

넥서스에서 나온 teps문법 관련 문항이네요..전 답이 d라고 생각했는데 c라 하네요..

관련 문장들을 네이버에서 겁색해 봤더니 더 혼란스럽습니다.. 관련사항을 알고 계시는 분의 설명이나 여러 의견을 듣고 싶습니다..

a series of paintings have been hung on the wall.
there are a series of bottlenecks in the way of this program.
your series of questions begs a series of answers.
there was a series of raps on the window.