Read the following article and select the best answer. Article: Do you know you could get sick just by walking past a building? The bacteria that cause Legionnaire's disease grow very fast in large modern buildings. They are carried by air through the buildings and outside. Legionnaire s disease is only one of the illnesses that are caused by "sick buildings". Some buildings can make you ill because their heating and air-conditioning are controlled . Air from outside cannot get in through the closed windows; This indoor air pollution is a growing problem. Legionnaire's disease may be the most serious result of sick buildings, but other unpleasant illnesses can appear.People who work in some buildings suffer from tiredness, feeling sick and headaches. Their offices look bright and clean, but invisible and dangerous bacteria are in the air around them. There is an answer to the problem. The air-conditioning and heating can be tested for bacteria. _ This is expensive to do, but the results are good. The people in the buildings no longer get sick, and everybody is happier. Question: We can tell that a building might be "sick" when _ . - the windows are all closed - bacteria can be seen in the air - the air-conditioning and heating don't work well - people in it have got different kinds of diseases
people in it have got different kinds of diseases