A:先升华,后凝华
B:先升华,后凝固
C:先汽化,后液化
D:先溶化,后凝固
—Will students go to school to have lessons?
—________.
等差数列{an}的前n项和记为Sn,若a3+a7+a11为一个确定的常数,则下列各数中也是常数的是( )