It still puzzles today’s scientists ______ some mammals produce their young ______ others lay eggs to produce their offspring( 后代)

It still puzzles today’s scientists ______ some mammals produce their young ______ others lay eggs to produce their offspring( 后代

1. A.
   that; while
2. B.
   why; while
3. C.
   how; when
4. D.
   what; as

It still puzzles today’s scientists ______ some mammals produce their young ______ others lay eggs to produce their offspring( 后代)

     A. that; while   B. why; while   C. how; when   D. what; as

It still puzzles today’s scientists        some mammals produce their young        others lay eggs to produce their offspring （后代）．

A．that; while            B．what; while          C．that; as           D．what; as