Kotlinx serialization

What is the output of the following code?

@Serializable
class City(val name: String, val population: Int,  @EncodeDefault val areaKm2: Double = 0.0)

val barcelona = City("Barcelona", 1700000)

val json = Json { prettyPrint = true }

val jsonBarcelona = json.encodeToString(barcelona)
println(jsonBarcelona)

a)

{
    "name": "Barcelona",
    "population": 1700000
}

b)

{
    "name": "Barcelona",
    "population": 1700000,
    "areaKm2": 0.0
}

c)

{"name":"Barcelona","population":1700000,"areaKm2":0.0}

d)

{"name":"Barcelona","population":1700000}