Question

A:"Has the secretary finished typing the report?"
B: "I _,but I'll ask"

1.doubt it
2.dont believe it
3.doubt so
4.dont think so

Answer 1

Answer:

4.I don't think so.....

Answer 2

A:"Has the secretary finished typing the report?"  

B: "I doubt it, but I'll ask"

Explanation:

When someone is not certain about the question asked, or the question asked is unlikely to happen, 'I doubt it' can be used. The uncertainty to the question "Has the secretary finished typing the report?" can be expressed using 'I doubt it' in the answer.

"I doubt it, but I'll ask" is the right answer for this question. I don't think so is used when one has evidence to their doubt about the happening of the event.