从环境保护的角度考虑，下列燃料中最理想的是（ ）
A．氢气
B．天然气
C．酒精
D．汽油
答案：A
Lots of trees ________in our city over the years. It is becoming greener and greener!
A．are planting
B．have planted
C．will be planted
D．have been planted
设关系R和S的元组个数分别为200和100，关系T是R与S的笛卡尔积，则T的元组个数是
A．400
B．10000
C．20000
D．90000
