Where would you expect the air pressure to be greatest in a valley?

The best answer would be:

The bottom of the valley.

If you'd like to know more, read on:

Air always pushes down on you. Think of it this way, the higher you go, the less air is above you, the lower you go the more air is above you. Although air is gas, it is still pulled by the gravity of the Earth. As you go closer to the surface of the Earth, the pull of gravity gets stronger and this happens to air particles too. So air closer to the surface of the Earth are more tightly packed, thus creating more pressure. On the flip side, as air goes further away from the surface of the Earth, they start to split further apart, creating less pressure.

i am going to guess that it is co2.

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."