How can I tell infomix which is the best index to use

This is basicly what I have one table with two fields indexed date_time and acct_number when I run the following querry it takes to long select * from table where date_time > "1998-04-02 00:00:00" and acct_number = 123; if I take the "and acct_number = 123" off it returns immediately any way I can tell informix to user the date_time index and not the acct_number index and still check the acct_number value note just sample data to make my point that it is using acct_number