If the acceleration is constant (negative or positive) the instantaneous acceleration cannot be
Average acceleration: [final velocity - initial velocity ] /Δ time
Instantaneous acceleration = d V / dt =slope of the velocity vs t graph
If acceleration is increasing, the slope of the curve at one moment will be higher than the average acceleration.
If acceleration is decreasing, the slope of the curve at one moment will be lower than the average acceleration.
If acceleration is constant, the acceleration at any moment is the same, then only at constant accelerations, the instantaneuos acceleration is the same than the average acceleration.
Constant zero acceleration is a particular case of constant acceleration, so at constant zero acceleration the instantaneous accelerations is the same than the average acceleration: zero. But, it is not true that only at zero acceleration the instantaneous acceleration is equal than the average acceleration.
That is why the only true option and the answer is the option D. only at constant accelerations.
Answer:
The resultant velocity is 
Explanation:
Apply the law of conservation of momentum
Where 
is the mass of the Luxury Liner = 40,000 ton
is the velocity of Luxury Liner = 20 knots due west
mass of freighter = 60,000
is the velocity of freighter = 10 knots due north
Apply the law of conservation of momentum toward the the west direction
So the equation would be
Substituting values
Where 
the final velocity due west
Making the subject
Apply the law of conservation of momentum toward the the north direction
So the equation would be
Where 
the final velocity due north
Making the subject
The resultant velocity is
Every person is different. But for a planet-wide overall average that roughly represents all human beings on Earth, the figures usually used are:
from 20 Hz to 20,000 Hz .
