16-20為題組
         Permafrost is any ground that remains completely frozen—0°C or colder—for atleast two years straight. It consists of soil, gravel, and sand, usually __16__ togetherby ice. These frozen areas are most common in regions with high mountains and nearthe North and South Poles.
         Permafrost soils store vast quantities of organic carbon, perhaps over twice theamount of carbon __17__ in the atmosphere. When permafrost is frozen, the organiccarbon can’t decompose, or rot away. As permafrost melts, microbes begin decomposingthis organic matter. This __18__ releases greenhouse gases like carbon dioxide andmethane into the atmosphere. These gases further warm the climate and thaw morepermafrost, thereby creating a vicious feedback cycle __19__ the permafrost carbonfeedback loop.
          Melting permafrost in the Arctic could also release hazardous chemicals, nuclearwaste produced during nuclear testing since the 1950s, and even mercury from decadesof mining. __20__ , the collapse of permafrost could release bacteria that are resistantto antibiotics or viruses that humans have never encountered. These newly unfrozenmicrobes could make humans and animals seriously ill.
【題組】16.
(A) bind
(B) binding
(C) to bind
(D) bound