Which Statement Illustrates a Policy Solution to an Inefficiency Caused

Which statement illustrates a policy solution to an inefficiency caused by an externality?

A) The city of Boston imposes rent control to address a homelessness crisis.
B) A minimum wage is imposed to bolster the incomes of low low-skilled workers.
C) The government offers free vaccines, since an immunized child cannot transmit disease to others.
D) The government provides national defense, since people can't be excluded from national defense, even if they don't pay for it.