In the symbol 4 2he, the superscript 4 is the for helium and the subscript 2 is the for helium.
a. atomic number; mass number
b. mass number; atomic number
c. atomic number; atomic mass
d. atomic mass; atomic number

Answer is: b. mass number; atomic number.

The superscript is mass number and the subscript is atomic number.

Atomic number(Z) is the number of protons, which is characteristic of a chemical element..  

Mass number (A) is the total number of protons and neutrons in a nucleus.

Helium (symbol: He) is a chemical element with two protons, two electrons and two neutrons.

Option (b) is the correct answer.

Explanation:

Mass number means the total number of protons and total number of neutrons present in an atom. Mass number is written at the top left of the symbol of atom.

Whereas atomic number means the total number of protons present in the atom. Atomic number is written at the bottom left of the symbol of atom.

For example, in , the number 12 denotes mass number of carbon atom whereas 6 denotes the atomic number of carbon atom.

Thus, we can conclude that in the symbol , the superscript 4 is the mass number for helium and the subscript 2 is the atomic number for helium.

answer: they produce more slow than unicellular organisms.