第一节 语法和词汇知识（共15小题；每小题1分，满分15分）
21．After the robbery, the shop installed a complicated alarm system as an insurance further losses.
A．for B．from C．towards D．against
22． to nuclear radiation, even for a short time, may produce variants(变异体)of genes in human bodies.
A．Being exposed B．Having exposed
C．Exposed D．After being exposed
