Once we have________some knowledge, things do not become more difficult, but comprehensible.
Teachers are now aware of the difficulty that some students have __________ attention.
—I made it at last. I am on top of the world!— __________
Liu Xiang trained hard before the Asian Games in Guangzhou, and it __________. He won the gold medal in 110 meters' hurdles.
I can't __________ to lose the papers because they contain a lot of confidential information.
The USA __________ on several countries in the past years, which reminds us that a nation must be strong if it __________.