—I wonder ______you’ll water this kind of flower .—Every other day .

As is often pointed out, knowledge is a two-edged weapon which can be used ________ for good or evil．

—Mom，dad is smoking again!—Well，let him cut it ________ gradually since old habits die hard.

I think you’ve got to the point where a change is needed , _____, you’ll fail .

He was __ twenty when he was gone.