True regarding excitation contraction coupling in smooth muscles is: (A) Presence of troponin is essential, (B) Sustained contraction occurs with high calcium concentration, (C) Phosphorylation of actin is required for contraction, (D) Presence of cellular calcium is essential to cause muscle contraction
Answer is D. Troponin is needed for skeletal muscle contraction, not smooth muscle. Sustained contraction occurs due to ‘latch’. 
Myosin needs to be phosphorylated rather than actin for smooth muscle contraction.