瘦肉精化学名称（）是一种（）。
参考答案：盐酸克伦特罗；β-受体激动剂
--- How long has your mother ______ Beijing?
--- About two weeks. But she will come back tomorrow.
A．been to
B．been in
C．gone in
D．gone in
J6的“6”表示经纬仪观测一测回方向中误差为6″。（）
