The answer is the first option. The one about doing your best. trust me I read the book today!

Since the sound travels from the submarine to the object AND back, it actually travelled 3625x2=7250m.

Speed of sound: 1450m/s
Explanation:
Law of conservation of momentum states that in an isolated system when two objects collide with each other then total momentum before and after the collision is equal.
Thus, we can conclude that the law of conservation of momentum states that the total momentum of interacting objects does not change. This means the total momentum before a collision or explosion is equal to the to momentum after a collision or explosion.
It can be both flat or it can be when you have new eyeglasses on and you look down it makes you think the ground looks like that but its not
Answer:
80 meters high
Explanation:
The velocity of the balloon would be g*t (I won't calculate, but will us this later)
We know that the kinetic energy at the bottom equals the potential at the top.
KE = PE
1/2 * m * v^2 = m * g * h
1/2 * m * (g * t)^2 = m * g * h (substitution)
1/2 * m * g^2 * t^2 = m * g * h
1/2 * g * t^2 = h (simplification by dividing the commons between both sides)
h = 1/2 * 9.81 * 4^2
h = 78.48 m (roughly 80 m)