Final answer:
Aircraft west of a certain point must be cleared northbound at even altitudes according to the Even-Odd Rule for altitude separation.
Step-by-step explanation:
Aircraft west of a certain point must be cleared northbound at even altitudes. This is because the aviation industry uses a standard called the Even-Odd Rule for altitude separation. According to this rule, planes flying west of a specific point are assigned even altitudes (such as 2000 ft, 4000 ft, etc.), while planes flying east of that point are assigned odd altitudes (such as 3000 ft, 5000 ft, etc.). This helps ensure safe separation between aircraft.