书名：笛藤新TOEIC必考单字600
页数：P133 第5题
题目：A supplier who has chronic trouble _____ his obligations to a customer
will quickly lose customers.
(A)fulfill
(B)fulfills
(C)fulfilling
(D)fulfillment
疑问：答案是C,请问为什么不是B?
谢谢!