If an expression is the difference of two squares, it will follow this format:

where a and b are integers or variables.
1)

Since the expression follows the format, it's a DoTS.
2) Since the binomial has an addition sign, it isn't a DoTS.
3)

The second term isn't an integer, so the expression isn't a DoTS.
4)

Since the expression follows the format, it's a DoTS.
Therefore, the first and fourth expressions are differences of two squares.
Answer: A
Step-by-step explanation:

48 cookies from 2 cups of flour, rate = 48 cookies / 2 cups = 24 cookies / cup
60 cookies from 3 cups of flour, rate = 60 cookies / 3 cups = 20 cookies / cup
If the rates were proportional, the rates would have the same answers.
Therefore the rates are not proportional.