在等差数列{an}中，已知a1+a3+a5=18，an-4+an-2+an=108，Sn=420，则n=（ ）。
20
He devoted most of his time to his research, believing his hard work would___sooner or later.
A．put off
B．take off
C．come off
D．pay off
在墙体类型划分中，按墙的方向又可分为（）。
A.围墙和隔墙
B.承重墙和非承重墙
C.纵墙和横墙
D.外墙和内墙
