________volcanoes now for many years, I am still as enthusiastic about my job as the first day I first______.

A．Studied, started                        B．Have studied, started

C．Having studied, started                   D．Having studied, starting

During the Middle Ages, a conventional artist was not interested in showing nature and people _____they really were.

A. who     B what    C. like     D. as

Without the ‘ greenhouse effect’, the earth ____ be about thirty-three degrees Celsius cooler than it is.

A．should            B．would            C．will              D．must

All the scientific evidence ______ that increasing use of carbon dioxide ______ the rise of the earth’s temperature.

A．show; result in                         B．shows; result from

C．shows; results in                        D．shows; results from

It is not clear ____the effects of global warming will be.

A．how             B．which            C．that              D．what

It is not until the disease has progressed to AIDS ____a person begins to look sick.

A．who             B．that              C．where            D．before

It rained that day and ___ the baseball game was called off.

A．however          B．still              C．so               D．consequently

It is high time that we ____ the amount of carbon dioxide in our daily life.

A. reduce     B reduced    C will reduce   D reduces

Please tell me how global warming _____. I am still in the dark. 

A．came by          B．came out          C．came to           D．came about

The medicine bought at a high price turned out to have no _____. 

A．influence         B．result             C．effect            D．risk