Which of the following statements is NOT true according to the passage?
A.
In the past, people sometimes plugged their ears to fight against the offending noise.
B.
An active noise-cancellation system follows the principle of a wave being flattened by meeting its exact opposite.
C.
The first active noise-cancellation system was made in the 1930s.
D.
Active noise-cancellation systems are now available on the market.