Which of the following are correctly punctuated and capitalized? Check all that apply.

This policy began in the $1970$s$.$ The population of China was increasing at a rate that was not sustainable.

This policy began in the $1970$s the population of China was increasing at a rate that was not sustainable.

This policy began in the $1970$s; The population of China was increasing at a rate that was not sustainable.

This policy began in the $1970$s; the population of China was increasing at a rate that was not sustainable.