In paragraph 2, the word noticed is closest in meaning to .

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 questions.
Amphibians are an animal group that includes frogs, toads, salamanders and newts. The animals live in and out of the water. The number of amphibians on our planet is declining rapidly in both local mass extinctions and population crashes. This is a cause for concern as many scientists believe that humans are responsible.
Since the 1980s, scientists have noticed a decline in amphibian populations. Although many animals are affected by humans, amphibians have been hit particularly hard. Some scientists believe this is because of their two-stage life cycle. Unlike most animals, amphibians live their lives in two distinct stages. The first is aquatic and the second is terrestrial. This means amphibians are sensitive to environmental changes in the water and on land. They also have permeable skin, which means that toxins and chemicals can easily get into their bodies.
At first, some scientists did not believe that human pollution was the cause of changes in amphibian populations. These scientists believed that every species on Earth goes through natural cycles and changes and that there was not enough long-term data to prove that humans were the problem. In recent years, this has changed. Almost all biologists are concerned about the decline in amphibian populations. They are worried not only because many amphibians may go extinct, but also because these extinctions will affect other plants and animals in ecosystems. They believe that the decline in amphibian populations is a warning to humans to stop polluting and clean up the Earth.