The underlined word "they" in the last sentence of the first paragraph refers to

A．Some animals and plants depend on each other for existence.
B．Some animals and plants develop their relationship easily.
C．Some plants depend on each other for food.
D．Some animals live better together.

A．People have done nothing to get rid of the tsetse flies.
B．People are trying to find out ways to get rid of the tsetse flies.
C．No ways can be found to get rid of the tsetse flies.
D．It won't be long before people get rid of all the tsetse flies in Africa.