Period of an ideal simple pendulum = 2π √(L / G)
1.87 = 2π √ (L / 9.81)
Divide each side by 2π : (1.87 / 2π) = √ (L / 9.81)
Square each side: (1.87 / 2π)² = L / 9.81
Multiply each side by 9.81 : L = (9.81) (1.87 / 2π)² = <em> 0.869 meter</em>
Choice 'D' is the closest one.
Recall that to compute for the emf of a circuit given current and inductance, we must recall that

where I is the current (A), M is the mutual inductance (h), and t is the time (ms). Since the current must not exceed 80.0 V, we have



From this, we see that it must take at least 0.35 ms so it doesn't exceed 80 V.
Answer: 0.35 ms