副词 up and down:
moving backward and forward along a given course
alternately upward and downward
- All over the ship the bulkheads sweated a film of oil and water.
- The landscape is made up of low, rolling hills.
- She watched the gentle rise and fall of his chest as he slept.
- He put up a notice about the change in price.
- New words and meanings keep the English language in a state of flux.