Final answer:
Among the options given, B. 3 miles per hour is the correct answer as it represents a unit rate. Option A ($5.99 for 1/2 pound), C (21 miles for 3 gallons), and D ($60 in 4 hours) do not represent unit rates as they do not measure quantities in relation to a single unit.
Step-by-step explanation:
The question requires understanding unit rates, which are ratios that express the quantity of one unit in relation to one unit of another measure. The options given are:
- A. $5.99 for 1/2 pound
- B. 3 miles per hour
- C. 21 miles for 3 gallons
- D. $60 in 4 hours
Now, let's determine which option represents a unit rate:
- A is not a unit rate because the cost is for 1/2 pound, not per one pound.
- B is a unit rate because it represents how many miles can be traveled in one hour.
- C is not a unit rate; it would be a unit rate if it were 7 miles per gallon (21 divided by 3).
- D is not a unit rate since it represents how much is earned in 4 hours, not per one hour.
Therefore, option B, 3 miles per hour, is the correct answer as it is the only option that represents a unit rate.