Choose all of the following that accurately describe contractile characteristics of smooth muscle.

a) contraction strength is graded in proportion to the ca2+ concentration of the cytosol.
b) a basal level of ca2+ in the cytosol of some smooth muscles results in a low-level of cross-bridge activity called smooth muscle tone.
c) some inputs to a smooth muscle membrane increase contraction strength, while other inputs decrease contraction strength
d) twitch contractions are "all-or-none," meaning they are the same strength regardless of stimulus strength.
e) a membrane action potential precedes contraction in all types of smooth muscle cells.