In the meanwhile, the quality of facial tissues was constantly improving, because more manufacturers went into the business of making tissues, and each manufacturer strove to make his product better than his competitors’.

A．the supply was quickly exhausted
B．its price went up
C．the manufacturers tried to improve their quality
D．more manufacturers went into the business of making tissues

A．the ice can easily just slip into the ocean
B．volcanoes caused the ice to melt
C．melting just in the Northern hemisphere would have been impossible
D．researchers often use the computer models help their research work