2.5与x的3倍的和是（ ）；a的4倍减去0.8，差是（ ）。
2.5+3x；4a-0.8
小儿听觉发育完善的年龄是（）。
A.3岁
B.4岁
C.5岁
D.6岁
E.7岁
Although he is only a _____ boy. He knows a lot.
A．five year old
B．five-year-old
C．five-years-old
D．five-year-olds
