Argument organization: משחק ותרגול

A tax on plastic bags (1) A tax on plastic bags is an effective way to protect the environment. (2) In countries that introduced such a tax, the use of single-use bags dropped sharply within a year. (3) Some economists object that the tax hits low-income families hardest. (4) However, the cost is easily avoided by bringing a reusable bag, which many shops give away. What is the function of the third sentence in this argument?