It is well known that biology is ______ science about living things.
The old photo reminded me of the happy hours_____ we spent together.
The door locks_____ when you leave the house, so you don’t have to remember to lock it.
Believe it or not, everything comes with_____ price; there is no such_____ thing as free lunch.
---Currently it is difficult to find a good job in the fierce job-hunting market.---_____.But highly-qualified graduates are always in great demand.
The first decade of the 21st century _____ the breathtaking and non-stop development of China.