The teacher won't write ________ chalk in the future.
A:in
B:on
C:with
D:by
我国北京周口店发现的“北京猿人”和“山顶洞人”分别属于( )
Tom had to______ the invitation to the party last weekend because he was too busy.
如图甲所示,一辆搬运砖头的独轮车,车箱和砖头所受总重力G=1200N,推车时人手向上的力F应为____ N.如图乙所示的动滑轮,支点为O,则动力臂LF是阻力臂LG的____ 倍.
园林工人给移栽后的绿化树“挂吊瓶”,补充水和无机盐,以提高成活率.“挂吊瓶”的针头应插入到茎的( )