Question

________ you have made a promise, you should carry it out．

1. A.
   Until
2. B.
   Once
3. C.
   For
4. D.
   As soon as

Answer 1

B
until, for引导从句时，常不住于句首。本题意为“一旦你许诺，你应该履行诺言”，应选择once，即选项B。once与as soon as的异同点：两者都可作连词用，表示“一……就”，但所强调的部分不同。once(＝from the moment that)强调条件，引导条件状语从句；as soon as(＝at the moment that)强调时间，引导时间状语从句。如：①Once you begin, you must continue．一旦开始你就得继续下去。②I'll return it as soon as I can．我将尽早将它还给你。