Scientists think that the continents ____ always where they ___today.
---When was Tom bitten by a snake?---It was on a Sunday _______ he was taking a trip to the mountainous area.
When this new kind of virus broke out at first in Mexico, no one had the least idea that it would _______ so rapidly throughout the world.
---Is the new job a real challenge for him?---Well, _______ difficulties he meets,I believe he will never lose heart.
“___________ race” means that two people form a group with their on-leg tied and they will walk for contest with three legs.
Listening to loud music at rock concerts _________ caused hearing loss in some teenagers.