Select the muscular sheet which is flat when it contracts and helps in breathing.

The correct option is B
Diaphragm is a muscular sheet which plays a vital role in the mechanism of breathing. During inhalation, the diaphragm contracts and becomes flat. This increases the volume in the thoracic cavity and creates a low pressure region which results in air gushing into the lungs. Since, the diaphragm is flattened, it also provides more space for the lungs to expand.