汽车在平直的公路上以20 m/s的速度行驶,当汽车以5 m/s2的加速度刹车时,刹车2s内与刹车6s内的位移之比为[ ]
A.1:1
B.3:4
C.4:3
D.3:1
答案:B
钢梁上的存水处应加泄水孔,泄水孔直径应大于80mm。
My aunt has a _____ boy.[ ]
A. five-years-old
B. five-year-old
C. five years old
D. five-years old