Each morning, millions of us turn on our radios and televisions for the weather forecast. Do the cloudy skies mean rain? Will rising temperatures bring a ___21___ to melt snow and ice? Once we hear the forecast, we decide what clothes to wear and ___22___ to carry an umbrella or not.

___23___, though, weather forecasts are ___24___ incorrect. Yes, though the accuracy of forecasts has improved ___25___ in recent years, predicting the weather is a fascinating mixture of art and science that is far from ___26___. What is involved in predicting the weather, and how reliable are weather forecasts?

Today weathermen have some advanced tools ___27___, the most basic of which measure air pressure, temperature, humidity, and wind.

Predicting the weather, however, is not all hard science. The computers can only give some descriptions of the behavior of the atmosphere. Furthermore, even an accurate forecast for a large area may not take ___28___ the effect of local area on the weather. So some ___29___ of art is also necessary. Here is where a weather forecaster comes in. He uses his experience and judgment to determine what value to ___30___ on the data he receives. This allows him to make a more accurate forecast.

A.whether

B.what

C. if

D.how