Physicists have made new discoveries that： challenge our （）theories of the universe.

单项选择题Afteranumberofdisagreementswiththecommittee，thechairmandecidedto（）.

A.resign 
B.retire 
C.retreat 
D.withdraw

单项选择题Itisreportedthatthereisnobetter（）formother’smilk.

A.alternative 
B.exchange 
C.equivalent 
D.substitute