Any projectile always has a constant horizontal acceleration of
a. 9.81 m. s^2
b. 0 m/s^2
c. -9.81 m/s^2
d. it is not constant.

B.

Explanation:

When we talk about projectile motion, we have to deal with parabolic motion, which has two components: vertical and horizontal.

In the case of Earth, the acceleration is due gravity, which is constant and directed downwards (vertically) and have a value of .

Now, in the case of the horizontal component, the acceleration will always be zero.

This is because we are dealing with parabolic movement, the x-component of the velocity remains the same (hence, there is no acceleration), while the y-component always change because it is affected by the acceleration due gravity that acts verticaly.

d. air pressure decreases as altitude increases.