Climate refugees are people who must leave their homes and communities because of the effectsofclimate change and global warming. Climate changeis caused by natural events, such asvolcanic eruptions, as well as human activities. Climate change__(16)__ many times since Earthwas formed billions of years ago. Global warming is the most recent period of climate change. Humanactivities like burning fossil fuels and cutting down forests__(17)__ global warming because theyrelease greenhouse gases. Greenhouse gases trap heat in the atmosphere.        Rising temperatures__(18)__ with global warming cause glaciers and ice caps to melt. Thiscan cause flooding and make sea levels rise. Rising temperaturesalso lead to droughts anddesertification-the transformation of __(19)__ land to desert. Some of these effects, such as sealevel rise, can put land completely underwater, __(20)__ it uninhabitable. Other effects, suchas drought, make it impossible for people in the region to support themselves.
【題組】16. ( )
(A) happens
(B) happening
(C) has happened
(D) had happened