One of the reasons why the long-term trend of prices bas been downwards is that__________.
A. technological innovation can promote social stability
B. political instability will cause consumption io drop
C. new farming and crop technology can lead to overproduction
D. new sources are always becoming available
What is least known about El Nino. according to the passage？
A. The climate changes caused by El Nino.
B. The areas influenced by El Nino.
C. How often El Nino happens.
D. How El Nino is formed.
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