Simple present/progressive and Simple past/ progressvie

Front

While they__________Paris last year, thousands of tourists saw Leonardo da Vinci's Mona Lisa at the Louvre.

Back

were visiting

Front

At one time the telephone system in the United States was a monopoly: different companies didn't ___________ offer telephone service.

Back

use to

Front

Jurors often deliberate for many hours before they __________ a decision.

Back

make

Front

The number of cases of asthma ______ these days, perhaps partly because more people own cats.

Back

is increasing

Front

Several people who refused to leave, ______ on Mount St. Helens when it erupted in 1980.

Back

were still living

Front

Nowadays, the average American routinely __________ about 20 hours of TV per week.

Back

watches

Front

Scientists now _______ that it is possible to clone organisms.

Back

know

Front

What did George Balanchine do regarding the New York City Ballet before 1948?

Back

didn't establish

Front

Before Pasteur developed his theories, people __________ that life could arise spontaneously.

Back

used to think

Front

Where is the mistake? When  (A) oxygen combines  (B) with hydrogen, (C)it sometimes is creating(D)  water. 

Back

D