"The challenge is if certain types of people are doing something, it's difficult for other people to break into it," said Po-Shen Loh, the head coach of last year's winning U.S. Math Olympiad team.

去年获胜的美国数学奥林匹克代表队的总教练罗博深说：“挑战在于，如果某些类型的人正在做某事，其他人就很难成功打入其中。”

youdao

The fact is, if you think JSF development is difficult you're probably not doing it right, and fortunately that's pretty easy to fix.

实际上，如果您认为 JSF 开发很难，可能是您没有用对，幸运的是，这很容易改正。

youdao

Going through difficult times with someone special is better than doing it alone.

同一个特别的人一起度过困难时期要比孤独一人好很多。

youdao

While doing the work out, it is difficult and I feel tired to the extreme with a temptation to stop, because no one is watching.

CNN: Interview with K.J. Choi

It's difficult because doing nets is nothing like being out in the middle.

BBC: Keeping the chin up

So to get any kind of idea as to who is doing this, it's very difficult.

NPR: Sectarian Divisions Complicate Iraqi Security Training