If three real numbers a,b and c are in harmonic progression, then which of the folllowing is true ?

If three real numbers a, b, c are in Harmonic Progression, then which of the following is true?

If, for positive real numbers x, y, z, the numbers x+y, 2y and y + z are in harmonic progression, then which one of the following is correct ?

If a, b and c are three real numbers, then which of the following is NOT true ?

Which of the folllowing number are prime? 27

Which of the folllowing number are prime? 37

Which of the folllowing number are prime? 19

Which of the folllowing number are prime? 83

Which of the folllowing number are prime? 57

Which of the folllowing number are prime? 204

Which of the folllowing number are prime? 139