there isn't much ___ i can doa.which b.what c.that为什么选C?

which:哪一个
what:什么
that:那个
整个句子的意思是：这不是很难，我可以做到（"这"!!）
还有前边是there所以选that

定语从句

that

推荐首诗，相信你看了就明白了！
There isn't much that I can do,
but I can share my bread with you,
and sometimes share a sorrow, too..
There isn't much that I can do,
but I can sit an hour with you,
and I can share a joke with you,
and sometimes share reverses, too..
There isn't much that I can do,
but I can share my flowers with you,
and I can share my books with you,
and sometimes share your burdens, too..
There isn't much that I can do,
but I can share my songs with you,
and I can share my mirth with you,
and sometimes come and laugh with you..
There isn't much that I can do,
but I can share my hopes with you,
and I can share my fears with you,
and sometimes shed some tears with you..
There isn't much that I can do,
but I can share my friends with you,
and I can share my life with you,
and oftentimes share a prayer with you

这个是定语从句 much是先行词而that的从句是修饰much的,所以不能选what
而且有much的句子只能用that不能用which
还有something ,nothing,the one等等都只能用that