1. Desertification is a form of land degradation, by which the once______ land becomes desert. A. barren B. fertile C. naked D. eroded
2. Peter Debye developed a method for bringing substances to the lowest temperatures ______. A. known to science B. known in science C. known by science D. known for science
【补全句子练习】 1. We are all ______ ______her safety. 我们都担心着她的安全． 2. ______ ______ all the money that I should pay you. 把我要付给你的钱加起来。
