—Sorry, but I _________ it for only half an hour. I haven't finished it.
A:lend; bought
B:borrow; have bought
C:borrow; have had
D:lend; have had
代数式3a+4b可以表示不同的实际意义,试举实例说明:____
当x=-2时,二次根式5-2x的值为( )
目前人类面临的三个问题是____ 、____ 和____ .