The World Health Organization says reducing salt or sodium use can reduce your risk of heart disease, kidney failure or stroke. But researchers say the risk would be reduced even more if the amount of dietary potassium was increased at the same time.
The World Health Organization says high blood pressure affects one billion people worldwide. It leads to many deaths or permanent disabilities. High blood pressure, also known as hypertension, is called the silent killer because there are few apparent signs. Many studies have shown that reducing salt or sodium in the diet    can lower the risk of stroke and other health problems.
Graham MacGregor is with the Barts and London School of Medicine and Dentistry. He and other researchers have examined earlier studies and organized their own.
“When you’re on the high salt intake you always have some extra salt in you and a slightly greater volume of blood. And that’s what puts up the blood pressure. I mean, if you wanted, an analogous thing would be really a central heating system. If you put more water into a closed system, the pressure will go up.”
He says the body does need salt, about less than half a gram each day. However, people in developing countries are eating about eight to 10 grams a day. Professor MacGregor says lowering salt can do a lot to reduce hypertension, but increasing potassium in the diet also helps. People in industrial nations eat about three grams of potassium a day through diet. Professor MacGregor says experts believe eating about four grams a day is the best choice.
Health officials have expressed concern about developing countries with growing economies. Those nations are increasingly turning to a western diet with its salty, sugary and fatty foods. Officials are predicting a sharp rise in heart disease and related problems.
Which of following has nothing to do with having extra salt in you？

A．heart disease

B．kidneyfailure

C．stroke

D．toothache

The writer uses “a central heating system”to_________.

A．give examples	B．introduce a new thing
C．compare the two things	D．contrast the two things

According to the passage，which of the following statements is true？

A．The body needs salt, about three grams each day.

B．Eating about four grams potassium a day is the best choice.

C．Lowering potassium can do a lot to reduce hypertension.

D．Reducing sodium in the diet canhigherthe risk of stroke.