Antigen Receptor Signaling Pathways Are Initiated by the Action of a Src-Family

Antigen receptor signaling pathways are initiated by the action of a Src-family kinase. In T cells, the predominant Src-kinase is Lck. In resting T cells, Lck is maintained in an inactive state by allosteric interactions involving multiple domains of the enzyme. When T cells are treated with a small molecule inhibitor of the tyrosine kinase Csk, TCR signaling is initiated even in the absence of a ligand to stimulate the TCR. This occurs because:

A) Csk phosphorylates Lck in its kinase domain, leading to Lck activation.
B) Csk phosphorylates ZAP-70, maintaining ZAP-70 in an auto-inhibited state.
C) Csk phosphorylates the ITAM motifs in the TCR $\zeta$ chain, leading to ZAP-70 recruitment.
D) Csk phosphorylates and activates the membrane tyrosine phosphatase CD45.
E) Csk phosphorylates the C-terminal negative regulatory tyrosine in Lck.