A, trampoline athletes are only affected by gravity when they reach the highest point in the air, and the acceleration is gravity acceleration, which is in weightlessness, so A is wrong;

B, the high jumper is in a state of complete weightlessness at the moment of crossing the crossbar, only affected by gravity. The acceleration is equal to the acceleration of gravity, so it is weightless, so B is correct;

C, the weightlifter stops at the highest point when lifting the bell, and the ling is in a static state and the force is balanced, so C is correct;

D, the size of inertia is only related to the mass of the object, and the run-up of long jumpers is not increasing their inertia, so D is wrong.

So: BC.