例句與造句

1. Then in 1921, Potts, who was a Superintendent of Signals for the Police Department, installed his first automatic signal light system in 15 towers.
2. Also, in DST the Null set is considered to have zero mass, meaning here that the signal light system exists and we are examining its possible states, not speculating as to whether it exists at all.
3. It's difficult to find signal light system in a sentence. 用signal light system造句挺難的