how to disable B frame of HEVC?

Hi, 

I run sample_encode on windows. I set the command arguments as follows: 

h265 -i D:\yuv\rpd_720p_slow.yuv -o output.h265 -w 1280 -h 720 -b 10000 -nobref -x 1 -u speed -p e5400a06c74d41f5b12d430bbaa23d0b

I expect hevc encoder output I P P P P P P...., but it's output is I BBBBBB. and the number of reference frame is not 1.

I saw the document Intel® Media Server Studio 2016 - HEVC Decoder and Encoder Release Notes, it says: 

 H.265 MAIN SP profile is not supported in current implementation.
 Real-time encoding not guaranteed.

My question is :

Is p frame supported by Hevc encoder ? Or how can i set the number of reference frames of B frame to 1?