一对杂合子的黑毛豚鼠交配，生出4只豚鼠，它们的表现型及数量可能是 [ ]
A．全部黑或全部白
B．三黑一白或一黑三白
C．两黑两白
D．以上任何一种
答案：D
—Could you tell me _______?
—It's twenty minutes by underground.[ ]
A. how can I get to your school
B. how much it costs to get to your school
C. how far it is from your home to your school
D. how long does it take me to get to your school
信号是对调车或行车与有关人员发出指示（）的命令。
