The topic sentence of the paragraph is; Opponents say that too little research has been done on the effects of GMOs on humans and on the environment.
A topic sentence as the name implies is one which summarizes the main idea of a paragraph. It is the first sentence in a paragraph in most cases. Otherwise known as the main idea as it the writer’s position on that topic, it encapsulates or organizes an entire paragraph.
However, in this case, it is the second sentence.
Read more on topic sentence;
https://brainly.com/question/1910346
#SPJ1