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.

Expanding the planet's protected natural areas to safeguard vanishing forests and other ecosystems, and the species they protect, is unlikely to be effective on its own as human encroachment into reserves grows, scientists warned Tuesday. A study by Cambridge University researchers, which looked at thousands of conservation areas in more than 150 countries, found that, on average, protected designation is not reducing human encroachment in vulnerable areas. Both chronic underfunding of efforts to protect the land, and a lack of engagement with local communities that live there are hurting conservation efforts, they found.

Creating protected areas is "a type of intervention that we know can work, we know is absolutely essential for conserving biodiversity, at a time in this world's history where it has never been under higher pressure," said lead author Jonas Geldmann. "But despite that we are seeing that some of our protected areas are not managing to mitigate or stop that increasing pressure," said Geldmann, of the University of Cambridge Conservation Research Institute.

One-sixth of the globe now falls within protected areas, the study noted. Those include national parks, nature reserves and wilderness areas, according to the International Union for Conservation of Nature's World Database on Protected Areas. Such protected areas are vital for preserving diverse ecosystems, and helping to curb climate change by conserving carbon-sequestering forests and other vegetation. The United Nations Environment Program estimates protected areas hold 15% of the carbon stored on land.

"Protected areas are one of the most important things that we can do to stem the loss of biodiversity and to help solve the climate crisis," said Andrew Wetzler, managing director of the U.S.-based Natural Resources Defense Council's nature program. "The destruction of natural habitat is the single biggest driver of extinction."

(Source: https://www.voanews.com/)

The word “stem” in paragraph 4 refers to _______.