After many ______(实验), he finally succeeded in finding the solution to the difficult problem.
Attempts to find a _______________(解决办法)have failed.
The building was completely________________(毁坏)in the earthquake.
Come to see me if it is ________________(方便的)for you.
Unfortunately,what the research center spent five years doing had no _______________(实际的)use.
Professor Smith accepted the invitation without ________________(犹豫)