The question that puzzles them is they can finish the work on time.
A. what it is that
B. what is it that
C. how is it that
D. how it is that
网友回答
A. what it is that
B. what is it that
C. how is it that
(答案→)D. how it is that
解析：句意：困扰他们的问题是他们究竟怎样才能按时完成工作。此处is之后是表语从句，表语从句由强调句型构成，相当于：it is how that they can finish the work on time.这里how是连词置于句首。故选D。