“USA? Britain? Which country is____(good) to study in?” We often hear such discussion. As China opens its door, it is a dream for many Chinese students to study abroad. They want to learn about the outside world.

    It's true that studying abroad can help students develop____(they). Their foreign language skills will____ (improve) and it may be easier to find jobs.

    But there are problems____ should be considered. Language is the first. Students must spend a lot of time____ (learn) another language.

    Students must also learn____ (live) without parents' care and deal with all kinds of things they didn't have to do before, like looking after themselves. There are ____ (report) about Chinese students abroad running into an ocean of difficulties____ giving up in the end. When they have to take care ____ themselves, it is hard for students to study well.

    Finally, studying abroad brings a heavy burden to the family. For most Chinese parents, the cost of studying abroad____ (be) very high. But is it worth it?

A、B两列简谐横波均沿x轴正向传播，在某时刻的波形分别如图甲、乙所示，经过时间t（t小于A波的周期T_A），这两列简谐横波的波形分别变为图丙、丁所示，则A、B两列波的波速v_A、v_B之比不可能的是（   ）