A:①②③
B:①③④
C:①②④
D:②③④
There are forty students in Tom's class. of them comes from Beijing.
已知:a-3+b+4=0,计算ab的值.
计算:(3+2)(3﹣2)=( )
下图是蚯蚓的外形图,请据图回答下列问题。