A works with a chemicals import&export company,but A__for this ihdustrial fair since A is on leave

题目

A works with a chemicals import&export company,but A__for this ihdustrial fair since A is on leave
选项：A.has worked B.works C.has been working D.is working,
这个专四题遇见到两次,但答案不同,老师也比较纠结.有选C 也有选D 的,请问如果专四真考到了到底选哪个啊?还有,since意思是“自从”还是连词“因为”,详细点!谢啦!
希望各位大师尽快答复.急需答案.谢啦啊.

答案