---We cant be successful we keep on working hard.-I agree with you.A. ifB. becauseC.

问题补充：

---We can be successful we keep on working hard.

---I agree with you.

A. if

B. because

C. unless

D. until

答案：

A. if

B. because

(答案→)C. unless

D. until

解析：连词unless意为“除非……如果不……”，“除了……”，多引导一个否定意义的真实条件句，有时也可引导非真实条件句。