The wind chill index tells you
a.the temperature of the water during winter
b.the amount of moisture in the air
c.how wind speed affects heating needs for structures
d.how wind and temperature affect your body in winter

The correct answer is D.how wind and temperature affect your body in winter. Basically, it's how you perceive the cold weather on your skin regardless of what the temperature actually is. It's usually lower and that's why you have forecasts say that it will be for example 0, but it will feel like -5 degrees Celsius.

In the area that she observed, the water level fell 13.64mm over 6.4 years, for an average annual rate of change of:
-13.64 ÷ 6.2 = -2.2 mm/ year. 

The difference between how much average water levels rose and how much water levels in her part of the river fell is:
1.8 -(-2.2) = 4 mm/year

During the 6.2 year period: 
4 mm/year * 6.2 = 24.8 mm