What is the main idea of the mention passage?

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions from 36 to 42

When corals are mentioned, many people think of rock-life forms in the sea filled with colorful fish. But corals are made up of hundreds of thousands of small organisms which live and feed like any other sea life. For the past five years, researchers in Hawaii and Australia have been engineering corals inside a lab to see if they could better resist the effects of climate change. They say it is now time to see how their creations perform in nature. The scientists say climate change linked to human causes has led to warming oceans that can harm sea life. They say if the more heat-resistant corals they developed do well in the ocean, the method can be used to help save suffering and dying reefs.

The team tested three methods for making corals that would be strong and healthy in nature. One was the method of selective breeding. This method involves scientists choosing parents with desirable characteristics for reproductive purposes. A second method subjected the corals to increasing temperatures to condition them to be able to survive in warm ocean environments. The third involved making changes to the algae that provide corals with necessary nutrients. The leader of the project, University of Hawaii researcher Kira Hughes, said all the methods proved successful in the lab. She told The Associated Press that some scientists might worry that such methods go against the natural processes of nature. But with the planet continuing to warm more and more, she does not see any better options.

“We have to intervene in order to make a change for coral reefs to survive into the future,” Hughes said. When ocean temperatures rise, corals release algae that supplies nutrients and gives them color. This causes them to turn white, a process called bleaching. When this happens, corals can quickly become sick and die. But for years, scientists have been observing corals that have survived bleaching, even when others have died on the same reef. They are now centering on those healthy survivors and hoping to further increase their resistance to heat. Those corals were used as the parents for the newly created kinds.

(https://learningenglish.voanews.com)

Câu hỏi:

What is the main idea of the passage?

• 
  A.
  Scientists have created corals in lab to better resist climate change.
• 
  B.
  There are many causes leading to climate change and loss of coral reefs.
• 
  C.
  A new kind of algae helps to bring more nutrients for corals.
• 
  D.
  Bleaching, a process where coral reefs turn white and die.