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 |
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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 |