铬是提高不锈钢耐腐蚀的基本元素，当钢中含铬量达到（）%时，钢耐腐蚀性会产生突变。
A.5
B.9
C.12
D.18
参考答案：C
The new computers _____ to a village school as presents last month.[ ]
A. are given
B. gave
C. were given
电话机双音多频拨号方式中的双音频是指两个特定的单半频信号的组合来代表数字或功能。
