His writing is so confusing that it's difficult to make out ________ he is trying to express.

A．what is it

B．what is it that

C．what is that

D．what it is that

在2008年北京奥运会馆“鸟巢”周围种植了凸显北京特色的乡土树木--国槐和雄性毛白杨，“国槐和雄性毛白杨”所属的植物类群是（　　）

A．藻类植物

B．蕨类植物

C．裸子植物

D．被子植物