下列各式是最简二次根式的是( )
A:9
B:7
C:20
D:0.3
封开县境内新发现了大型的“钼矿”,金属钼可用于制造火箭、导弹等耐高温材料。如果钼铅矿(PbMoO4)中的铅元素为+2价,则钼元素的化合价为
( )
It's good for people to walk more and drive less. So I'd rather _______ half an hour's walk to work.