I took about two hours (小时) my homework.
A:do
B:to do
C:doing
在干燥的天气里,用塑料梳子梳头发时,头发会随着梳子飘起来,这是因为梳子和头发摩擦时分别带上____电荷,互相____的缘故.
化简:(1﹣ 1m+1 )•(m+1)=____.