A,因为前面是there is
是可数的,但是其复数形式和sheep一样.
例句：There are a lot of sheep.
sheep
[Fi:p]
n.
羊,绵羊,羞答答的人,胆小鬼,信徒
sheep
[Fi:p]
n.
sheep
羊,绵羊
a black sheep
败类；害群之马；不肖之徒
make sheep's eyes at sb.
愚笨地向某人献殷勤