When viewing a Quote, the sales representative wants to easily see how many discounted items are included in the Quote L

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When viewing a Quote, the sales representative wants to easily see how many discounted items are included in the Quote Line Items.What should a developer do to meet this requirement?

A. Create a trigger on the Quote object that queries the Quantity field on discounted Quote Line Items.
B. Create a Workflow Rule on the Quote Line Item object that updates a field on the parent Quote when the item is discounted.
C. Create a roll-up summary field on the Quote object that performs a SUM on the quote Line Item Quantity field, filtered for only discounted Quote Line Items.
D. Create a formula field on the Quote object that performs a SUM on the Quote Line Item Quantity field, filtered for only discounted Quote Line Items.