30. 下列何者為平滑肌(smooth muscle)收縮時所必需之分子，而其它肌細胞不需要的？
(A) 旋轉素(troponin)
(B) 攜鈣素(calmodulin)
(C) 鈣離子
(D) 肌動蛋白(actin)

Which of the following descriptions about muscle and skeletal system is NOT true? 
(A) The strength of a muscular contraction is determined by the number of neurons delivering action potentials. 
(B) Skeletal, cardiac, and smooth muscle all have transverse tubules. 
(C) A hydrostatic skeleton consists of fluid held under pressure in a closed body compartment. 
(D) Gap junctions provide direct electrical coupling between the cardiac muscle cells. 
(E) Calcium ions cause smooth muscle contraction by binding to calmodulin.