In the past, operations were difficult. Until the middle of the eighteen fifties, surgery was very dangerous. Many patients died after even the smallest operations.This was because bacteria entered the cuts in the patients" bodied and started infection. In some countries, up to 90 percent of patients died from infection after operations.In 1865, however, Joseph Lister, a British surgeon, found an answer to the problem. He used an "antiseptic" during and after operations. This killed the dangerous bacteria and most of his patients lived. Since then, surgeons have used antiseptics in all operations.Surgery has developed in many important ways since the day of Joseph Lister. Today, when patients go to hospital for an operation, they can expect the best treatment, in clean and hygienic conditions. In the past, up to 90 percent of patients died after operations mainly because ______.

A. bacteria entered the cuts in the patients" bodies and infection took place
B. the conditions in hospitals were bad
C. the skill of surgeons was not so good
D. there were no good medicine at that time