当肋间外肌和膈肌舒张时,气体经过的路线是( )
A:外界气体→食管→肺
B:肺→气管→口腔→外界
C:外界气体→气管→肺
D:肺→气管→鼻腔→外界
已知等差数列{an}的前n项和为Sn,且S10=55,S20=210.求数列{an}的通项公式;
There is going to _________ a storm.