According to a model regressing a firm's monthly sales (in thousands) of a product on the price of that product, the 90% confidence interval on the average monthly sales is [5.3, 7.7] if the price is $6. Which of the following could be 90% prediction interval for a single month of sales if the price is $6?

a. [4.4 , 8.2]

b. [4.4 , 8.6]

c. [5.9, 7.1]

d. [6.2, 8.4]