志愿者精神是什么？
参考答案：
奉献、友爱、互助、进步
During the period of recent terrorist activities, people _____ not to touch and unattended bag. [ ]
A. had always been warned
B. were always being warned
C. are always warning
D. always warned
编出下列字母式子简便运算的公式，并各举一例证明．
a-b-c=______a÷b÷c=______
例：______ 例：______．
