17)what element is this how do you know?

That element is Helium.

Explanation:

We can see in the image above that the element has two negative particles (which are the electrons), two positive particles (which are the protons) and two neutral particles, without electrical charge (which are the neutrons).

If we look for a periotic table and observe the electron numbers of the elements and the atomic number (number of protons), we realize that the only elementp that has two protons and two electrons is Helium.

b. the carbon cycle ensures living things have a source of hydrogen