Use the postulates of kinetic theory of gases

According to kinetic theory:

• Gas molecules are in continuous random motion.
• Collisions between molecules and with the walls are perfectly elastic.
• Between collisions, molecules move in straight lines with constant velocity.
• Molecular speeds are distributed according to the Maxwell distribution, so only a small fraction of molecules have very high speeds.

Check each option

Option A: collisions are always elastic

This is a direct postulate of kinetic theory.

Therefore, A is correct.

Option B: heavier molecules transfer more momentum to the wall of the container

Momentum of a molecule is $p = mv$ but kinetic theory does not say that heavier molecules always transfer more momentum.

At a given temperature, $\frac{1}{2}mv^2 \propto T$ so heavier molecules generally move more slowly. Hence larger mass does not automatically mean more momentum transfer in general.

Also, pressure depends on average translational kinetic energy, not simply on molecules being heavier.

Therefore, B is incorrect.

Option C: only a small number molecules have very high velocity

From Maxwell's distribution of molecular speeds, most molecules have intermediate speeds, while only a small fraction lie in the high-speed tail.

Therefore, C is correct.

Option D: between collisions, the molecules move in straight line with constant velocity

This is again a standard assumption of kinetic theory, since intermolecular forces are neglected except during collisions.

Therefore, D is correct.

Final derived answer

The correct options are: $\boxed{A,\ C,\ D}$

Comparison with stored correct answer

Stored correct answer: $A, C, D$

My derived answer matches exactly.