Read the passage "Superweeds" in the box.
SUPERWEEDS
Due to new technology, scientists have been able to alter the DNA in corn, soybeans, and cotton to make them resistant to the chemical herbicides used to kill weeds. Consequently, millions of acres of United States farmland have been regularly sprayed with herbicides, leading to the development of a herbicide tolerance in weeds. Producers must now use larger amounts of herbicides and find new, stronger chemicals to fight off the herbicide-tolerant weeds.
What was the initial factor that led to the development of herbicide-resistant weeds?
a
genetic cloning
b
spontaneous reproduction
c
genetic engineering
d
artificial selection