Answer:

Explanation:
Let's use projectile motion equations. First of all we need to find the travel time. So we are going to use the next equation:
(1)
Where:






In this case:

Because the dog jumps horizontally
Let's asume the gravity constant as:


Because when the dog reach the base the height is 0


Now let's replace the data in (1)

Isolating t:

Finally let's find the horizontal displacement using this equation:

Replacing the data:

Alpha particles have both high mass and low velocity, but it would be the former that determines why they can't penetrate. An alpha particle is made of two protons and two neutrons, which is quite a large mass for a particle. This large mass keeps it from penetrating too deeply into anything, since it will essentially "get stuck" when it comes too near to another atom.
So the answer, I believe, is B. Alpha particles have high mass.