They were discussing this question,and were almost building up some weak structure of hope on his prolonged absence,when they heard him on the stairs.
A.
他们议论着这个问题,几乎在迟迟不起了结构脆弱的希望,这时,他们听见他上楼的声音。
B.
他们正在讨论这个问题,他长久不回来,使他们几乎失去了信心,但这时,他们听到了他往楼上走的脚步声。
C.
他们讨论着这个问题,并且对他迟迟不回,几乎产生了一线希望,这时突然听见他上楼的脚步声。