An important difference between single-unit and multiunit smooth muscle is the a. numerous gap junctions in single-unit smooth muscle, which allow many cells to work together as a sheet. b. longer actin and myosin filaments in multiunit smooth muscle, which allow coordination of contraction. c. ability of single-unit fibers to change into multiunit fibers when advantageous. d. closely controlled individual fibers in single-unit smooth muscle to allow fine control and graded contractions by selective activation.