Final answer:
To find the cost of building a storage house, multiply the area of the floor by the cost per square foot.
Step-by-step explanation:
To calculate the cost of building the storage house, we first need to find the area of the floor. The area of the floor is given by the formula length x width. So, the area of the floor is 12 feet x 10 feet = 120 square feet. Since the cost of building per square foot is $70, the total cost of building the storage house is 120 square feet x $70/square foot = $8,400. Therefore, the correct answer is option d, $8,400.