Two forces act in opposite directions on an object. If no other forces act on the object, then it will accelerate

if the net force is greater than zero

Explanation:

If two forces acts in opposite directions on an object and no other forces acts on the object, it will accelerate if the net force acting on it is greater than zero.

Acceleration is brought about the net force acting on a body if it is greater than zero.

When the net force on a body is zero, a body will not accelerate and will continue to remain in its state of rest of maintain a uniform unaccelerated motion.

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

any of the answers are correct.