Read the extract given below and answer the questions that follow:
Sadao knew that his education was his father's chief concern. For this reason he had been sent at twenty-two to America to learn all that could be learned of surgery and medicine. He had come back at thirty, and before his father died he had seen Sadao become famous not only as a surgeon but as a scientist. Because he was perfecting a discovery which would render wounds entirely clean, he had not been sent abroad with the troops. Also, he knew, there was some slight danger that the old General might need an operation for a condition for which he was now being treated medically, and for this possibility Sadao was being kept in Japan.
Dr. Sadao was not sent abroad with the troops because he was______.

Options

A. a scientist
B. working on a discovery
C. a doctor not a soldier
D. studying in America