An aircraft flies IFR in controlled airspace. How often must the altimeter system and altitude reporting equipment be tested?

1. 1Within the preceding 24 calendar months
2. 2Within the preceding 12 calendar months
3. 3Within the preceding 36 calendar months
4. 4Within the preceding 100 flight hours

Source: 14 CFR Part 91, § 91.411 Altimeter system and altitude reporting equipment tests and inspections.