While looking at the bottom surface of a leaf with a compound light microscope, a student notices pairs of cells with openings between them on the surface of the leaf. the main purpose of these openings and the cells that surround them is
1. removing excess sugars2. synthesis of carbon dioxide3. regulating gas exchange4. purification of water

3. regulating gas exchange

Explanation:

What the student is observing are the structures responsible for regulating gas exchange in vegetables. These structures are called stomata and allow the loss of water vapor (which leaves the plant into the atmosphere), and the entry and exit and carbon dioxide or oxygen, which as you may know are gases directly involved in photosynthesis.

Stomata are only opened (allowing small openings to be visualized in the plants with the help of a microscope), when the entry or exit of a gas is extremely important for the panta to maintain its homeostasis efficiently.

looks like the answer is     c     to me!

geologic time scale (for gradpoint) =)