It can be concluded from the passage that ______.
A．the positive and negative ions are always equivalent in the air
B．when winds such as the Mistral, Foehn, Hamsin or Sharav are blowing in certain countries, larger proportion of positive ions are found
C．some sensitive people feel sick or become mentally disturbed when a large number of negative ions are present
D．ionizers, according to some scientists, can generate both kinds of ions that make people feel good