Which one of the following is a sign of heart failure?

Heart failure is a condition in which the heart cannot pump as much blood as the body needs. This condition can begin with a heart attack or high blood pressure. The heart becomes weakened. Over time, it loses the ability to pump blood. A sign of heart failure is breathing difficulty. But people often mistake that for just a sign of aging.
     In the United States, doctors say about five million people have this major cause of sickness and death.  They say heart failure is the leading cause of hospital treatment for people aged sixty-five and older.  But these doctors say the problem of heart failure has yet to be fully investigated.
     Researchers at the Mayo Clinic in Minnesota did a study of the last twenty years. They expected to find  that heart failure has become more common. Hospital admissions for heart failure have increased. But they found that the percentage of the population developing heart failure has not changed. Instead, survival rates have improved.