Read the following passage on commuting, and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions from 51 to 60.

Many folk cures which have been around for centuries may be more therapeutic than previously suspected. A case in point is that of penicillin Alexander Fleming did not just randomly choose cheese molds to study when he discovered this very important bacteria-killing substance. Moldy cheese was frequently given to patients as a remedy for illness at one time Fleming just isolated what it was about the cheese which cured the patients.

In parts of South America, a powder obtained from grinding sugar cane is used for healing infections in wounds and ulcers. This usage may date back to pre-Colombian times. Experiments carried out on several hundred patients indicate that ordinary sugar in high concentrations is lethal to bacteria. Its suction effect eliminates dead cells and it generates a glasslike layer which protects the wound and ensures healing.

Another example of folk medicine which scientists are investigating is that of Arab fishermen who rub their wounds with a venomous catfish to quicken healing. This catfish excretes a gellike slime which scientist found to contain antibiotics coagulant that helps close injured blood vessels, anti-inflammatory agents, and a chemical that directs production of a glue-like material that aids healing.

It is hoped that by documenting these folk remedies and experimenting to see if results are indeed beneficial, an analysis of the substance be made, and synthetic substances be developed for human consumption.

In what way are cheese molds, sugar, and catfish slime similar? 

A. They eliminate dead cells

B. They heal wounds 

C. They fight bacteria

D. They cause blood clots

a)f(-2) =3*(-2)² -1=3*4-1=12-1=11

f(4)= 3*4²-1= 3*16-1=47

b)Ta có: 3x²-1=47

<=>3x²=48

<=>x²=16

=>x=4