It is the first time that I _______ this kind of moon cake.

—Must I turn off the gas after cooking? —Of course, you can never be ____careful with that．

There is no ____ that 100 years ago animal testing was cruel, but today animals in experiments are very well taken care of.

     private cars are bringing us convenience, they may also cause traffic accidents and pollution.

Only when you told me about it ________ what a silly thing I had done.