I _____ the dance performance many times, each giving me a new understanding. [ ]

A. watch

B. watched

C. have watched

D. am watching

采集烟气中氟化物样品时，当烟气中共存尘氟和气态氟时，需按污染源（）采样方法进行等速采样；当烟气中不含尘氟，只存在气态氟时，可按污染源（）采样方法进行采样。