So far, scientists ________ about 1.8 million living species(物种), and that ________ just a small num

So far, scientists ________ about 1.8 million living species(物种), and that ________ just a small number of what probably exists on Earth．

A.had named; was

B.had named; is

C.have named; has been

D.have named; is

答案及解析： D