Need the sum of all rows in column d where column b is a particular letter

[dropanddrive03]

Hi there, In the attached spreadsheet I need '$ Chart'!S2 to be the sum of '$ Tables'!D3:D73 but only for the rows where the value in column b is "c". I shouldn't be adding in any value where column b is U or N. This seems like it should be extremely easy, but it's fighting with me. I'd appreciate any help I can get.