Definition: without any success Level: C1
他们寻找丢失的文件，但徒劳无功。
They searched ___in ___vain for the missing document.
我们徒劳地等待着承诺的回复。
We waited ___in ___vain for the promised reply.
他试图说服她的一切努力都白费了。
All his attempts to persuade her were ___in ___vain.
