What output will program №13 produce

Consider the following Swift code and determine which lines will be printed to the console upon program termination.

import UIKit

class Alpha {
    deinit {
        print("A")
    }
}

class Beta {
    let a = Alpha()

    deinit {
        print("B")
    }
}

class Gamma: Alpha {
    let b = Beta()

    deinit {
        print("C")
    }
}

var instance: Gamma? = Gamma()
instance = nil