易燃固体的危险性标签的符号为（），底色为（）。
参考答案：黑色；白色红条
全向天线振子长度为（）
A、λ/2
B、λ/3
C、λ/4
D、λ/5
—Where is your father?
—He _________ Beijing on business. He will be back soon.
A. has gone to B. has been to C. has been in
