The students ______ busily when Miss Brown went to get a book she ______ in the office.[ ]

A. had written; left

B. were writing; has left

C. had written; had left

D. were writing; had left

在自然环境中，各种因素__________着生物，生物在生存发展中不断地__________环境，同时也__________和__________着环境。