Which of the following could replace ‘systems’ as used in the passage?

Directions: Read the passage carefully and select the most suitable option according to the corresponding number in the passage.

Electricity gives us shock, because it is an outside force that interferes (1) with the internal electricity our bodies' nervous systems generate (2).
To fully understand why the chance encounter (3) of these two electrical forces results in a shock to our systems (4), we must first understand the fundamentals (5) of electricity itself.
In scientific terms, electricity is considered a fundamental force, one that is extremely basic, and has been in existence (6) since the beginning of time.
Electricity comprises (7) positive and negative charges, opposite charges attract each other, and similar charges repel (8) each other.