Test center:
Special topic: Special topic of linear motion law.
Analysis: the frame of reference refers to the reference objects selected when studying the motion of objects or the object systems that do not move relative to each other; The choice of reference system is arbitrary. How to choose the frame of reference must be considered from the specific situation. Generally, we take the ground or objects on the ground as the frame of reference.
When the pilot was flying horizontally at high altitude, he found an "insect" falling freely beside him. As a result, the "insect" freely falls next to the pilot because of his horizontal speed, so it shows that the horizontal speed of the "insect" is the same as that of the plane. At this time, if the plane is selected as the reference system, the bullet remains relatively static with the plane in the horizontal direction, and only the bullet falls freely in the vertical direction, so B is correct.
So choose: B.
Comments: In order to study and describe the motion of objects, we introduce reference systems and choose different reference systems. The motion state of the same object relative to different reference frames may be different. Choosing a suitable reference frame can simplify the study of motion.