Your answer is wrong.scala

scala> import scala.collection.mutable.ArrayBuffer
import scala.collection.mutable.ArrayBuffer

scala>

scala> object T1 {
     | def x(ab: ArrayBuffer[Int]): Unit = {
     | ab += 10
     | }
     | def t1 = {
     | val a = ArrayBuffer(1,2,3)
     | x(a)
     | println(a)
     | }
     | def t2 = {
     | var a = ArrayBuffer(1,2,3)
     | x(a)
     | println(a)
     | }
     | }
defined object T1

scala> T1.t1
ArrayBuffer(1, 2, 3, 10)

scala> T1.t2
ArrayBuffer(1, 2, 3, 10)