People are in a constant search for different sources of energy. Today, we meet most of our energy needs by burning fossil fuels

People are in a constant search for different sources of energy. Today, we meet most of our energy needs by burning fossil fuels (16)— coal, oil and natural gas. (17)-—, one problem with fossil fuels is that when burned for energy, they release carbon dioxide and other gases. For instance, burning these fossil fuels releases large amounts of greenhouse gases, which are also released when they are retrieved from the earth. Namely, extracting the fuels and processing them (18)— their final forms require fossil fuels, and thus produce carbon dioxide. Another problem is that fossil fuels give a one-time-only burst of energy. Therefore, since the supply of fossil fuels is limited and will finish one day, civilisation will have to wait millions of years before any more (19)—. That is why people will have to (20)— a diversity of fuel sources to produce energy in the future. 16.
A) such as
B) contrary to
C) due to
D) according to
E) except for 17.
A) In other words
B) However
C) Otherwise
D) For example
E) Accordingly 18.
A) upon
B) towards
C) off
D) into
E) against 19.
A) had existed
B) exist
C) existed
D) will be existing
E) would have existed