```go
package main

import "fmt"

func isPrime(n int) bool {
    if n < 2 {
        return false
    }
    if n == 2 {
        return true
    }
    if n%2 == 0 {
        return false
    }
    for i := 3; i*i <= n; i += 2 {
        if n%i == 0 {
            return false
        }
    }
    return true
}

func main() {
    var t int
    fmt.Scan(&t)
    
testcase:
    for tc := 0; tc < t; tc++ {
        var k int
        var n string
        fmt.Scan(&k)
        fmt.Scan(&n)
        
        for i := 0; i < k; i++ {
            d := int(n[i] - '0')
            if !isPrime(d) {
                fmt.Println(1)
                fmt.Printf("%c\n", n[i])
                continue testcase
            }
        }
        
        for i := 0; i < k; i++ {
            for j := i + 1; j < k; j++ {
                num := int(n[i]-'0')*10 + int(n[j]-'0')
                if !isPrime(num) {
                    fmt.Println(2)
                    fmt.Printf("%c%c\n", n[i], n[j])
                    continue testcase
                }
            }
        }
    }
}
```