State the name of research postgraduate student among Professor Zhihua Zhou's research assistants.
research postgraduate student refers to type = 'RPG'; research assistant refers to the student who serves for research where the abbreviation is RA;
SELECT T3.f_name, T3.l_name FROM prof AS T1 INNER JOIN RA AS T2 ON T1.prof_id = T2.prof_id INNER JOIN student AS T3 ON T2.student_id = T3.student_id WHERE T1.first_name = 'Zhihua' AND T3.type = 'RPG' AND T1.last_name = 'Zhou'