user: Question: Which call sign has a Class of a, and a Frequency MHz of 88.7?
Columns: ['Call sign', 'Frequency MHz', 'City of license', 'ERP W', 'Class', 'FCC info']
Types: ['text', 'real', 'text', 'text', 'text', 'text']
Sample row: ['WVTW', 88.5, 'Charlottesville, Virginia', '1,000', 'B1', 'FCC']
SQL:

assistant: SELECT "Call sign" FROM "Table" WHERE "Class" = 'a' AND "Frequency MHz" = 88.7;