What can we learn about sun energy in the 20th century and 21st century?(本题2.5分)
A、 Solar output will reach its minimum in the 21st century.
B、 Solar output in the 20th century was the highest in history.
C、 Sunspot numbers in Cycle 24 has reached its peak in the 21st century.
D、 Sunspot numbers in Cycle 24 is less than half those in the 20th century. √
出自：西安石油大学大学英语I（下）
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