A planet is at equal distance from two stars, Star 1 and Star 2. If both stars have the same mass,

they will not pull each other with a gravitational force
they will pull the planet with equal gravitational force
Star 1 will pull the planet with a greater gravitational force than Star 2
Star 2 will pull the planet with a greater gravitational force than Star 1

In this situation both o the stars will pull the planet with equal gravitational force.

Explanation:

The reason behind this is that gravity works according to the mass an object has. Therefore if we have two corpses with the same amount of mass they are going to pull the objects with the same strength. Now, in our case, they have a planet between both of them. Thus, they are going to pull it to them with the same gravitational force because they have the same mass, and are at the same distance. If one of them had a bigger mass that would pull the planet with more gravitational force, as well as if that star was closer to the planet.

a. the production of carbon dioxide through the process of photosynthesis