The operations manager of a mail order house purchases double $(D)$ and twin$(T)$ beds for resale. Each double bed costs $$500$ and requires $100$ cubic feet of storage space. Each twin bed costs $$300$ and requires $90$ cubic feet of storage space. The manager has $$75,000$ to invest in beds this week, and her warehouse has $18,000$ cubic feet available for storage. Profit for each double bed is $$300$ and for each twin bed is $$150.$ The manager's goal is to maximize profits.

What is the storage space constraint?

$100D+90T18,000$

$500D+100T18,000$

$90D+100T18,000$

$300D+90T18,000$

$100D+90T18,000$