一辆汽车在水平路面上匀速直线行驶，阻力恒定为f. t1时刻驶入一段阻力为2f的路段继续行驶．t2时刻驶出这段路，阻力恢复为f.行驶中汽车功率恒定，则汽车的速度v及牵引力F随时间t的变化图象可能是 （ ）
The boy in red T﹣shirt is our new classmates，______？（ ）
isn't he
does he
is he
doesn't he
The new school ______ last year．（ ）
builds
built
was built
is built
My parents often tell me ______ hard．（ ）
study
to study
studying
studied
What's your brother doing now？He _____ in the room．（ ）
sings
sung
has sung
is singing
Who is ______，Tom or Dave？Dave．（ ）
tall
taller
tallest
the tallest
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