Final answer:
The correct rule of thumb for downspouts is at least one downspout for each 30 feet of gutter.
Step-by-step explanation:
In order to determine the number of downspouts needed for a given length of gutter, we need to find the rule of thumb. Based on the provided options, we can see that the rule of thumb is at least one downspout for each 30 feet of gutter. Therefore, the correct answer is C) 30 feet.