A。

no sooner…than…是惯用句型，其意为“刚……就”“一……就”。如：I had no sooner closed the door than somebody knocked. 我刚关上门，就有人敲门了。当no sooner置于句首时，其后的主句要用倒装语序，如：No sooner had she agreed to marry him than she started to have terrible doubts. 她刚同意嫁给他，就产生了种种可怕的疑问。