50.
(A) When the flight is boarding, it is time to say goodbye to your friends or family who mayhave come to see you off.
(B) When boarding the plane, your friends or family who have come to see you off may needtime to say goodbye to you.
(C) After having boarded the plane, your friends or family may have a hard time seeing you off.
(D) On boarding the plane, a passenger’s friends and family will be waiting nearby to see himor her off.