02年9月语法两问！！！

susan919

16.   Unique among bivalves, scallops swim extremely well, propelled by jets of water expelled while snapped the shell shut.

为什么选C啊～

16.   In the period between 1918 and 1939, various political, economic, and geographic

factors combined in determine the extent to which a country developed civil aviation.

为什么选 B？

谢谢

mildsevens

snapped->snapping,表主动关系

determine->determining，否则有两个谓语了