Find an expression for the induced electric field outside a solenoid (r > R) in which the magnetic field is changing at the rate dB/dt.

the answer is neither ball has a charge.

because if they were both positively charged, they will repel each other.

if they were positively and negatively charged, they will seek.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.