B at t2, the magnetic induction intensity is zero, but the magnetic flux change rate is the largest, so the induced current is the largest, and the magnetic flux passing through the metal ring is the largest. B correct;
C. During the time of t 1-t2, the change rate of magnetic flux increases continuously, so the induced current in the coil increases continuously. According to Lenz's law, the current direction in the coil is from F to C, and the magnetic flux passing through the ring increases outward according to the right-hand spiral law, so the clockwise induced current is generated in the metal ring according to Lenz's law. So, C is wrong.
D. During t 1-t2, the magnetic field in L increases, which can be determined by the law of coolness. L has to reduce its area to prevent the increase of magnetic flux, so it tends to decrease, so D is correct.
So choose BD.