In the following passage, some words have been deleted. Read the passage carefully and select the most appropriate option to fill in each blank.
A natural gas barrier in the upper atmosphere called the ozone layer (1.) _________ people and other living things from the sun's harmful ultraviolet (UV) radiation. Ozone exists in tiny concentrations (2.) _________ the atmosphere, but the majority (about 90%) is found in the stratosphere, a layer that is 10 to 50 kilometres (3.) _________ the surface of Earth. The ozone layer is essential to life on Earth because it (4.) ________ out the majority of the sun's harmful UV rays. In the 1970s, researchers (5.) __________ that the ozone layer was thinning.
Select the most appropriate option to fill in blank number 1.