As the saying goes, one is never too old to learn. After he _____, he will devote himself to gardening.  

A．retired

B．has retired

C．will retire

D．had retired

有以下程序 #include＜stdio.h＞ #define PT 5.5 #define S(x)PT*x*x main() { int a＝1，b=2；printf("%4.1f\n"，S(a+b) )； } 程序运行后的输出结果是 ( )

A．49.5

B．9.5

C．22.0

D．45.0