What is the MAIN difference between Cost of Goods Sold (COGS) and Operating Expenses (OpEx) on a company's Income Statement?

A.COGS corresponds to expenses that can be *directly* linked to individual products or services sold, whereas with OpEx there is not a direct relationship.

B.COGS can reflect only cash expenses, whereas OpEx may include both expenses paid out in cash in the current period, as well as expenses that are owed and which will be paid out in cash in the future.

C.COGS is almost always projected as a percentage of revenue, but OpEx is rarely projected that way since it can't be linked to individual units sold.

D.They're both expenses, but OpEx is almost always significantly bigger than COGS.