Mary was born in 1998 and she began to play guitar at the age of seven.
A:a
B:/
C:the
D:an
“复数a+bia,b∈R为纯虚数”是“a=0”的( )
用____记录降落伞从打开都下降到地面的时间。
用括号内所给的短语,将下列句子译成英语。