The rise in average temperature is due to the _____.

Read the given passage and answer the four questions that follow:-

All those people who say that the weather hasn't been normal recently are right. However, since there is really no such thing as normal weather, they probably mean something else. If they mean that average summer temperatures have been getting warmer, then they are certainly correct. In fact, it is not just the summer temeprature, but the general average temperature of the whole word that has been steadily moving up. The signs are everywhere. There are small signs. for example, butterflies in North America have moved about 60 miles north of where they used to live. There are also very large signs. Huge masses of ice in mountain glaciers and the Arctic are melting. The famous snow cap on Africa's Mount Kilimanjaro has almost disappeared. Actually, ice or snow doesn't really disappear, it turns into water, flows into the ocean and causes sea levels to rise. Why is this happening? One answer is that it could simply be part of a natural process. After all, there have been long periods of warmth in the past. So we could just be experiencing another warming trend. This kind of answer had more supporters in the past. What scientists now believe is that human activity is the cause. For more than two hundred years, humans have been gradually changing the atmosphere, mainly as a result of industrial pollution. We have created an atmosphere around the earth that, like a giant glass container, lets heat from the sun through and holds it inside.