By midnight the floor was wet with beer. One young technician slipped and bumped into another. They struggled, ________ trying to get the other in a head-lock.

A．every

B．all

C．everyone

D．each

凸性描述了价格和利率的( )关系。

A．线性测量

B．二阶导数

C．非线性测量

D．一阶导数