— Listen! Someone is knocking at the door. Who can it be?
— It _______ be Tom. He said he would visit me this evening.
A:can't
B:need
C:must
D:mustn't
如图,点C在线段AB上,点M、N分别是AC、BC的中点.
(1)若AC=8cm,CB=6cm,求线段MN的长;
(2)若C为线段AB上任一点,满足AC+CB=a,其它条件不变,你能猜想MN的长度吗?写出你的结论并说明理由;
(3)若C为直线AB上线段AB之外的任一点,且AC=m,CB=n,则线段MN的长 .
制作人口腔上皮细胞临时装片时,要用滴管在载玻片的中央滴一滴( )