libx264 can't disable B frame in pyav

[zhiweny1122]

I use a logitech usb camera and chrome browser to visit the janus server.
I use aiortc to pull the browser stream and the aiortc always fail.
the Pyav always create B frames when I use it in aiortc .
Can you tell me how to diable B frame in libx264 codec of pyav ?
Obviously ffmpeg can disable frame when converting video.
It doesn't work even though I set the pts and time_base to None.

pts 218112 vlastpts 0 dts 218112 kind audio
*** pts 97 vlastpts 0 dts 96 kind video
*** pts 219136 vlastpts 0 dts 219136 kind audio
*** pts 220160 vlastpts 0 dts 220160 kind audio
*** pts 102 vlastpts 0 dts 97 kind video
*** pts 221184 vlastpts 0 dts 221184 kind audio
*** pts 100 vlastpts 0 dts 98 kind video
*** pts 222208 vlastpts 0 dts 222208 kind audio
*** pts 223232 vlastpts 0 dts 223232 kind audio
*** pts 99 vlastpts 0 dts 99 kind video
*** pts 224256 vlastpts 0 dts 224256 kind audio
^C[aac @ 0x55ff1b7bf780] Qavg: 286.566
[libx264 @ 0x55ff1b7c72c0] frame I:1 Avg QP:20.14 size: 8587
[libx264 @ 0x55ff1b7c72c0] frame P:38 Avg QP: 5.65 size: 9357
[libx264 @ 0x55ff1b7c72c0] frame B:103 Avg QP:13.23 size: 651
[libx264 @ 0x55ff1b7c72c0] consecutive B-frames: 1.4% 4.2% 4.2% 90.1%
[libx264 @ 0x55ff1b7c72c0] mb I I16..4: 25.9% 63.9% 10.2%
[libx264 @ 0x55ff1b7c72c0] mb P I16..4: 0.8% 1.1% 1.2% P16..4: 24.5% 5.2% 4.5% 0.0% 0.0% skip:62.8%
[libx264 @ 0x55ff1b7c72c0] mb B I16..4: 0.0% 0.0% 0.0% B16..8: 11.8% 1.0% 0.2% direct: 0.9% skip:86.1% L0:42.0% L1:51.2% BI: 6.8%
[libx264 @ 0x55ff1b7c72c0] final ratefactor: 9.28
[libx264 @ 0x55ff1b7c72c0] 8x8 transform intra:48.4% inter:38.3%
[libx264 @ 0x55ff1b7c72c0] coded y,uvDC,uvAC intra: 51.4% 41.4% 30.4% inter: 6.5% 5.5% 4.1%
[libx264 @ 0x55ff1b7c72c0] i16 v,h,dc,p: 38% 21% 30% 11%
[libx264 @ 0x55ff1b7c72c0] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 41% 20% 25% 2% 2% 2% 3% 2% 3%
[libx264 @ 0x55ff1b7c72c0] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 46% 26% 15% 2% 3% 2% 2% 2% 2%
[libx264 @ 0x55ff1b7c72c0] i8c dc,h,v,p: 63% 14% 20% 3%
[libx264 @ 0x55ff1b7c72c0] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x55ff1b7c72c0] ref P L0: 83.4% 9.1% 6.0% 1.5%
[libx264 @ 0x55ff1b7c72c0] ref B L0: 89.2% 10.2% 0.6%
[libx264 @ 0x55ff1b7c72c0] ref B L1: 97.2% 2.8%
[libx264 @ 0x55ff1b7c72c0] kb/s:718.62