the task is difficult they managed to complete it on time.A. BecauseB. ThereforeC. AsD.

问题补充：

the task is difficult, they managed to complete it on time.

A. Because

B. Therefore

C. As

D. Though

答案：

A. Because

B. Therefore

C. As

(答案→)D. Though

解析：虽然任务难，但他们设法及时完成了。因此选D。