6. As there is less and less coal and oil, scientists are exploring new ways of making use of energy, such as sunlight, wind and water for power and fuel.
A. primary
B. alternative
C. instant
D. unique
网友回答
A. primary
B. alternative
C. instant
D. unique
解析：
Primary初等的；最初的；alternative可供替代的，可选择的；instant 立即的，即刻的； unique 单独一无二的，由常识可知，阳光、风力及水力都是不可能替代能源。