Why Does Health Insurance Become Less Expensive When the Size

Why does health insurance become less expensive when the size of the insurance group increases? (Cost Advantage)

A) Administrative costs are lower,more employees are contributing co-payments,the threat of high-risk individuals driving out low-risk ones is less
B) Insurance is less risky to provide,the threat of high-risk individuals driving out low-risk ones is less,administrative costs are lower
C) More employees are contributing co-payments,insurance is less risky to provide,administrative costs are lower
D) Insurance is less risky to provide,more employees are contributing co-payments,the threat of high-risk individuals driving out low-risk ones is less