Read the passage below. Noise pollution is defined as the "presence of unwanted sound.” One major source of noise pollution comes from transportation—namely, motor vehicles. Too much exposure to noise pollution has been linked to many serious side effects, such as hearing loss, stress, and sleep disturbance. Since noise pollution has been linked to many harmful effects, many people feel it is time to take a look at what measures can be taken to reduce noise pollution in the United States.

The beginning of which sentence indicates a cause-and-effect relationship?

“One major source of noise pollution”
"Noise pollution is defined as"
“These sounds may be considered”
“Since noise pollution has been linked”