Receptor Tyrosine Kinases Represent Critical Molecules Involved in Growth and Differentiation

Receptor tyrosine kinases represent critical molecules involved in growth and differentiation though phosphorylation of target substrates on tyrosine residues. Which structural feature is not common among all receptor tyrosine kinases?

A) An N-terminal extracellular ligand-binding domain
B) A single polypeptide
C) A cytosolic C-terminal domain with tyrosine kinase activity
D) A single transmembrane $\alpha$ helix