【填句补文】The black robin(旅鸫)is one of the world's rarest birds. It is a small, wild bird, and it lives only on the island of Little Mangere, off the coast of New Zealand. In 1967 there were about fifty black robins there; in 1977 there were fewer than ten. ________.
A
These are the only black robins left in the world.
B
Those that fail to meet the challenges will disappear early.
C
Both represent orders in the classification of life.
D
Detailed studies are going on, and a public appeal for money has been made.
