The classical pathway for complement activation is initiated by

The classical complement pathway for complement activation is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.

Explanation: The classical complement pathway typically requires antigen-antibody complexes (immune complexes) for activation (specific immune response), whereas the alternative pathway can be activated by C3 hydrolysis, foreign material, pathogens, or damaged cells.

After activation, a series of proteins are recruited to generate C3 convertase, which cleaves the C3 protein. The C3b component of the cleaved C3 binds to C3 convertase to generate C5 convertase, which cleaves the C5 protein. The cleaved products attract phagocytes to the site of infection and tags target cells for elimination by phagocytosis. In addition, the C5 convertase initiates the terminal phase of the complement system, leading to make appear the membrane attack complex. The membrane attack complex creates a pore on the target cell's membrane, inducing cell lysis and death.

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.