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
如图,要拧开一个边长为a=6cm的正六边形螺帽,扳手张开的开口b至少为多少?
下列加线字注音完全正确的一项是( )