A:this, book
B:that, book
C:these, books
D:those, book
小胖在深夜用手电光向夜空深处射去,27秒后手电光走了____千米?
函数f(x)=-x2+2x+3的单调递减区间是 ( )
图中①②是我国两个省区,它们的简称分别是