As a general rule, if pollution costs are external, firms will produce a. too much of a polluting good. b. too little of a polluting good. c. an optimal amount of a polluting good. d. an amount that cannot be determined without additional information.

External costs are those costs that are not directly accounted for by the producer, such as pollution costs in the environment. In our case, we are dealing with a polluting good that has an external cost associated with its production. Since these costs are not accounted for by the firm, it does not influence their production decisions, leading to unregulated production outputs.

When there are external costs associated with production, firms do not take these costs into account. They only consider their private costs, which are directly related to production, such as labor and material costs. Consequently, firms will produce at a level where their private costs are minimized, but the social costs (private costs + external costs) are not minimized. As a result, the production level with external costs can be inefficient from society's point of view.

We have four options to evaluate: a. too much of a polluting good b. too little of a polluting good c. an optimal amount of a polluting good d. an amount that cannot be determined without additional information As we have already established that the presence of external costs associated with polluting goods leads to inefficiencies in production, we can eliminate option "c" because the production level is not optimal due to external pollution costs. Option "d" can also be eliminated, as the exercise asks about the general rule, and we can determine the general production behavior of firms when faced with external pollution costs without additional information. Now, we are left with option "a" and option "b." When firms do not take external costs into account, they will produce at a level where their private costs are minimized, not considering the costs they impose on society. Since the external costs are not taken into account, the actual production level will be higher than the socially optimal level.

Therefore, as a general rule, if pollution costs are external, firms will produce too much of a polluting good. The correct answer is: a. too much of a polluting good.