问题描述：
The scientists wanted to know ＿＿＿when they felt confident.
A、why did they push out their chins
B、why they pushed out their chins
C、how did they push out their chins
D、how they pushed out their chins
A、why did they push out their chins
B、why they pushed out their chins
C、how did they push out their chins
D、how they pushed out their chins
问题解答：
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