What are the three possible causes of the 767's fuel pump alarm answer?

In the article, the "Crash of Flight 143," the fuel pump alarm of 767 was most likely caused by the failure of the pump, the clogging of the fuel line, and the running dry of the left tank. 

Air Canada Flight 143, in the Air Canada's Boeing 767 aircraft, was a scheduled passenger flight between Montreal and Edmonton,Canada when the aircraft ran out of fuel on July 23, 1983 as it was 12,500 off the ground. Luckily, Boejing 767 made a safe emergency landing at a former Royal Canadian Air Force base in Gimli, Manitoba. There were a total of 69 survivors, no casualties, and 10 non-fatally injured during the incident. 

D.successful because there were no legal protections for unions.