It is not a problem ______ we can win the battle;it's just a matter of time.( )
whether
why
when
where
Had the government limited the sales of cars much earlier, traffic jam and pollution _______so serious now.
____today, he would get there by Friday.
Had they known what was coming next, they _________second thoughts.
Had you taken it more seriously, you ________ such a silly mistake.
_________, the world would be more harmonious.