— ______ did she do yesterday?
— She made a cake. ( )
A:Why
B:When
C:What
变量控制是化学实验中常用的设计方法.在探究影响不同固体物质在溶剂中溶解限量的因素时,不需要控制的变量是( )
已知命题P:∀x∈R,cosx≤1,则¬P是( )