In the following essay, each blank has four choices. Choose the best answer and write down on the answer sheet.
Microwave communication uses high-frequency (26) waves that travel in straightlines through the air. Because the waves cannot (27) with the curvature of the earth, they can be (28) only over short distance. Thus, microwave is a good (29) for sendingdata between buildings in a city or on a large college campus. For longer distances, the waves must be relayed by means of "dishes" or (30) . These can be installed on towers, high buildings, and mountain tops.

A．reflex

B．distort

C．bend

D．absorb

以下关于顺序加锁法及其缺点叙述错误的是______。

A)该方法对数据库中事务访问的所有数据项规定一个加锁顺序
B)每个事务在执行过程中必须按顺序对所需的数据项加锁
C)维护对这些数据项的加锁顺序很困难，代价非常的大
D)事务按照固定的顺序对这些数据项进行加锁比较方便