13) E

Read the question carefully! It says “when blood first enters that muscle”
When blood first enters the muscle, it’s oxygen concentration would be higher than the muscle and the carbon dioxide concentration would be lower than in muscle to allow diffusion of both gases.
The first column should have low and the second column should have high. We can rule out A,B and C.

We know that the process involved is diffusion. Hence, we can rule out F since it says osmosis.

We know oxygen concentration in muscle would be low since it uses oxygen for respiration. So, D is incorrect, leaving us with E.