维持蛋白质二级结构稳定的主要因素是( )。
A．离子键
B．氢键
C．疏水力
D．范德华力
E．以上都是
参考答案：B
—What were you doing at this time yesterday?
—We _______ in the classroom.
A．read
B．were reading
C．are reading
D．was reading
汽车维修标准化体系分三个层次，分别是基础和通用标准，专用维修技术标准，维护、修理和检测设备。
