One Monday morning, Paul and his classmates were in science laboratory for their practical chemistry lesson. The students were going to work in pairs to do an experiment. Before they began, the teacher gave them this description of the different stages of the experiment.
Stage 1: Prepare the equipment: a test tube, a crucible, a Bunsen burner and tongs.
Stage 2: Weigh 5 grams of baking soda (sodium bicarbonate) and put it in a crucible.
Stage 3: Heat 10 ml of hydrochloric acid (盐酸) in a test tube.
Stage 4: Pour the warm acid onto the baking soda and continue to heat the mixture.
Result: The acid reacts with the bicarbonate（碳酸氢盐）to form water, carbon dioxide gas and salt. The hotter the mixture, the quicker the reaction. Continue to heat the mixture until the water evaporates, leaving the salt in the crucible.
Paul and his partner followed the instructions and set up their equipment. Paul went to the cupboard to get a bottle of hydrochloric acid. He found that there was only one bottle in the cupboard so he took that. There was no label on the bottle and Paul didn’t check with the teacher if it was the right solution.
He measured the liquid and poured it into a test tube. Using tongs to hold the test tube, he heated it over the Bunsen burner. That’s when things started to go wrong. The liquid in the test tube was not hydrochloric acid. When it was heated, it formed a thick cloud of white gas. Soon the room was full of this strong smelling white gas.
All the students started coughing and their eyes hurt. The teacher immediately opened the windows and ordered the students to leave the laboratory at once. She realized that the liquid was a crylamide (丙烯酰胺) and that it is poisonous.
Fortunately, nobody was injured in the incident. However, it taught the students and the teacher a good lesson.
What lesson did the incident teach the students and the teacher?

A．Never have a bottle without a label in the chemistry lab!

B．Obey your teacher’s instructions in the Chemistry lab.

C．A crylamide can’t take the place of hydrochloric acid.

D．Finding something unusual happened, you have to leave the laboratory at once.

What substance were the students making in the experiment?

A．Water

B．Carbon dioxide gas

C．Salt

D．Poisonous gas

Why did Paul make the mistake?

A．He went to the cupboard to get the bottle by himself.

B．He found that there was only one bottle in the cupboard so he took that.

C．There was no label on the bottle.

D．Paul didn’t check with the teacher that it was the right solution.

Why did the teacher send the students out of the classroom?

A．Becausethe room was full of this strong smelling white gas.

B．Because all the students started coughing and their eyes hurt.

C．Becausethe liquid was a crylamide and that it is poisonous.

D．Because someone was injured in the incident.