Helen is ______ office worker in a big company near Xinjiekou.
A:a
B:an
C:the
D:/
①质量和体积适中 ②与恒星的距离比较适中
③处于比较安全的宇宙环境之中④昼夜更替的周期较适中
计算:-52﹣|2﹣2|﹣-273.