我从来不能开别人的玩笑，因为我无法保持严肃的表情。

I can never play jokes on people because I can't ___keep ___a ___straight ___face.

在严肃的会议上，他努力保持面无表情。

He tried to ___keep ___a ___straight ___face during the serious meeting.

当有人讲一个非常搞笑的笑话时，很难保持不笑。

It's hard to ___keep ___a ___straight ___face when someone tells a hilarious joke.