136. Read the following passage, then answer the question below.

When Alexander Fleming noticed in 1928 that mold on a laboratory dish had killed the bacteria around it, he opened the age of antibiotics. Within two decades, infections that had once been death sentences became routine to treat. Yet Fleming himself warned in his Nobel lecture that careless use of penicillin could teach microbes to resist it.

His warning has proved accurate. Bacteria reproduce so quickly that a single resistant individual, produced by random mutation, can give rise to millions of resistant descendants in a few days once an antibiotic has killed its competitors. This is natural selection in action, but compressed into a timescale that medicine can observe. Bacteria can also exchange genes, including those for resistance, with unrelated species, so a trait that arises in one organism may spread across an entire hospital.

Overuse accelerates the process. Antibiotics are prescribed for viral infections such as colds, which they cannot cure, and they are widely used in livestock to promote growth. Even patients who take the correct drug may stop as soon as they feel better, leaving the hardiest bacteria alive. The consequences are sobering: the World Health Organization estimates that drug-resistant infections contribute to well over a million deaths each year.

New drugs are not an easy solution. Developing an antibiotic takes a decade and costs hundreds of millions of dollars, yet because new antibiotics are kept in reserve and used sparingly, companies earn little from them. As a result, many large pharmaceutical firms have left the field. Some researchers argue for new economic models in which governments pay for the availability of an antibiotic rather than for the number of doses sold, a design that would reward companies for developing drugs that are rarely used. Whether such an arrangement can be adopted widely remains uncertain.

The word 'which' in the third paragraph refers to