青藏高原比四川盆地太阳辐射强大的原因是

①离太阳近②青藏高原比四川盆地太阳高度角大 ③云层薄、云量少       

④地势高，空气稀薄  ⑤日照时间长  ⑥夜晚大气逆辐射弱

A．①②③

B．④⑤⑥

C．③④⑤

D．①②⑥

— What do you think of Spain?

— Well, considering its diverse cultures, it is an attractive country, _____ worthy of a visit. [ ]

A. it

B. one

C. this

D. that