When and where to go for the on-salary holiday ___________ yet.
A:are not decided
B:have not been decided
C:is not being decided
D:has not been decided
①____;
②____.
(Ⅰ)求证:a、b、c成等差数列;
(Ⅱ)若 B=π3,S=83 ,求b.
地球上最大的纬线圈是
____