Jenny's Chinese _______ a lot since she _______ to China two years ago.
A:improved; came
B:has improved; has come
C:improved; has come
D:has improved; came
下列叙述中,符合质量守恒定律的是( )
已知两点A(1,1),B(5,4),若向量a→=(x,4)与AB→垂直,则实数x=____
实验中的下列操作正确的是( )