Answer:
The mass number is defined as the total number of protons and neutrons in an atom. Consider Table below which shows data from the first six elements of the periodic table. Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus.
Explanation:
Sea level because air a much thinner at higher altitudes
External fertilization in animals usually occurs in water or in damp areas in a process called spawning.