He finally contracted the disease. He was aware of the importance of preventive measures only then.

A. But for his awareness of the importance of preventive measures, he could have contracted the disease.

B. Hardly had he been aware that preventive measures were essential when he contracted the disease.

C. Not until he was aware of the importance of preventive measures did he contract the disease.

D. Only after he had contracted the disease was he aware that preventive measures were essential.