A:①④
B:②③
C:①②④
D:①③④
-- Anna, could you lend me this book, please?
-- I'm sorry. I it for only two days, and I haven't finished it yet.
一箱苹果,4个4个地数,最后余下1个;5个5个地数,最后余下2个;9个9个地数,最后余下7个.请设计一种算法,求出这箱苹果至少有多少个?
设等比数列{bn}的前n项和为Sn,若S10=3S5,则S15:S10=( )