A group of birds 是单数还是复数？

A group of birds 是单数还是复数？

踢上来一脚

我觉得这个a group of birds应该是复数，一群鸟。

可是如下的例子中却说是单数，这不对吧？

 GWD-13-11

A South American bird that forages for winged termites and other small insects while swinging upside down form the foliage of tall trees, the graveteiro belongs to the ovenbird family, a group of New World tropical birds that includes more than 230 species and that are represented in virtually every kind of habitat.

1. graveteiro belongs to the ovenbird family, a group of New World
                       tropical birds that includes more than 230 species and that are

2. graveteiro belongs to the ovenbird family, a group of New World
                       tropical birds that includes more than 230 species and  is

3. graveteiro belongs to the ovenbird family, a group of New World
                       tropical birds that include more than 230 species and  is

4. graveteiro, which belongs to the ovenbird family, a group of New World tropical birds that includes more than 230 species and that are

5. graveteiro, which belongs to the ovenbird family, a group of New World tropical birds that includes more than 230 species and is

the answer is B

我觉得是单数啊，关键词是GROUP

多谢楼上。

单独拿出来 a group of birds，会有两种意思：一个是一群鸟（a bunch of birds），一个是鸟的集合,群类（a family of birds）.

在这里group 是family的同位语，所以指的是群类