为什么‘Write a T if the sentence is t-rue.´而‘Write an F if

为什么‘Write a T if the sentence is t-rue.´而‘Write an F if it is false.´?F不是元
为什么‘Write a T if the sentence is t-rue.´而‘Write an F if it is false.´?
F不是元音,为什么用an?求原因,
最好专业一点