According to the passage, "dwindle" means______.

A．The rate of killing has been accelerating.
B．The US government forbids imports of both raw and finished ivory.
C．They realised that the killing of elephants is a serious threat to their tourist business.
D．African people advocated an ivory ban.

A．African Elephants and the Ivory Trades.
B．A Bid to Save the Elephants.
C．The Poachers.
D．Elephants in Danger.