I can only do it for you ______

I can only do it for you ______ I'm paid in advance.
A、just as
B、in that
C、now that
D、on condition that
F、 
【正确答案】：D
【题目解析】：just as 正像；in that 因为；now that 既然；on condition that 如果，条件是句意：我能为你做这件事，条件是我得先得到报酬。