33. A grocery store carries 2 brands of cat food, Brand A and Brand B, both sold only in 20 1b bags. The regular price per bag of Brand A cat food is $4 more than the regular price per bag of Brand B cat food, which is $b per bag. This week, each bag of Brand A cat food is on sale for 50% off its regular price. Which of the following is an expression for the sale price per bag of Brand A cat food in terms of the regular price per bag of Brand B cat food?