CAPE Biology Unit 2 · 2011 · Paper 1 · Question Q21
The first sound in the cardiac cycle is caused by the
- (A)closing of the semi-lunar valves
- (B)opening of the semi-lunar valves
- (C)closing of the atrio-ventricular valves
- (D)opening of the atrio-ventricular valves
The answer is shown once you've answered.
Practise this question