6. What is the moment of inertia I for a uniform ring of radius R and mass M about an axis (perpendicular to the plane of ring) at its rim? (A)0.5 MR2； (B) MR2, (C) 1.5 MR2； (D) 2 MR2； (E) None.

4. If the energy needed to send an object (initially at rest on the earth's surface) to a maximum height H is a half of that for sending it into an orbit at the same height, what would be the value of H in terms of RE (the radius of the earth)? Ignore the earth's rotation. (A) 1/4； (B) 1/3； (C) 1/2； (D) 2/3； (E) None.

6. What is the moment of inertia I for a uniform ring of radius R and mass M about an axis (perpendicular to the plane of ring) at its rim?
(A)0.5 MR²；
(B) MR²,
(C) 1.5 MR²；
(D) 2 MR²；
(E) None.

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