Which SELECT statement returns the number of unique email addresses for messages in the spam folder?

SELECT COUNT$($DISTINCT folder$)$ FROM email WHERE spam $!=$ sender;

SELECT DISTINCT COUNT$($sender$)$ FROM email WHERE folder $=$ 'spam';

SELECT COUNT$($DISTINCT sender$)$ FROM email WHERE folder $=$ 'spam';

SELECT DISTINCT COUNT$(\text{'}$spam$\text{'})$ FROM email WHERE sender $=$ folder;