Second, we make a [very] (conservative) assumption that we are looking for a life form (that is [pretty well] like us), [since if it differs radically from us] we may [well] not recognize it as a life form, quite apart from whether we’re able to communicate with it.What’s the trunk(主干） of the main sentence（主句）?
选项：

A:SVO：We make a(n) assumption
B: SVO：We are looking for a life form
C: SV：It differs
D: SVP：That is like us