[单选题] "当专用无线系统中心交换控制设备或传输链路出现故障时,各基站能够自动切换至( )。
A.集群模式
B.单站集群模式
C.广播模式
D.紧急呼叫模式"

Malls are popular places for Americans to go. Some people spend so much time at malls that they are called "mall rats". Mall rats shop until they drop in the hundreds of stores under one rood.
People like malls for many reasons. They feel safe because malls have police stations of private security guards. Parking is usually free, and the weather inside is always fine. The newest malls have beautiful rest areas with waterfalls and large green trees.
The largest mall in the United States is the Mall of America in Minnesota. It covers 4.2 mil- lion square feet. It has 350 stores, eight night clubs, and a seven-acre park! There are park- ing spaces for 2,750 cars. About 750,000 people shop there every week.
The first indoor mall in the United States was built in 1965 in Edina, Minnesota. People like doing all their shopping in one place. More malls were built around the country. Now, malls are like town centers where people come to do many things, th
A. They feel safe because malls have police station of private security guards.
B. They can be served free meals after doing shopping.
C. They can do about everything at malls.
D. The weather inside is always fine.