What are good questions ______a conversation?
A:start
B:to start
C:starting
D:started
用如图所示的实验测定空气中氧气的含量,下列操作会使测定结果偏大的是 ( )
如图,把矩形ABCD对折,折痕为MN,矩形DMNC与矩形ABCD相似.则矩形DMNC与矩形ABCD的长与宽之比是____