Working out the order in which the individual components in a signaling pathway act is an essential step in defining the pathway. Imagine that two protein kinases, PK-B and PK-X, act sequentially in an intracellular signaling pathway: - When either kinase is completely inactivated, cells do not respond to the normal extracellular signal. - Cells containing a mutant form of PK-B that is permanently active respond even in the absence of an extracellular signal. - Doubly mutant cells that contain inactivated PK-X and permanently active PK-B respond in the absence of a signal. A. In the normal signaling pathway, does PK-B activate PK-X or does PK-X activate PK-B? Explain your reasoning.