Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_name_56 (erp_w INTEGER, class VARCHAR, frequency_mhz VARCHAR)

### Question ###
Name the sum of ERP W for class of d and frequency mhz of 98.7

### Accurate SQL ###

SELECT SUM(erp_w) FROM table_name_56 WHERE class = "d" AND frequency_mhz = 98.7