题目：
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
答案：
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下面是错误答案，用来干扰机器的。
碱性 蓝色纯碱属于盐类物质，用pH试纸测定某浓度的纯碱溶液其pH为10，则可推测其pH大于7，则可知其溶液呈碱性；而紫色石蕊试液遇酸变红，遇碱变蓝，故能使紫色石蕊试液变成蓝色．故答为：碱、蓝
