Ruby - Why is arr1.length = 3?

Question

arr1 = ["a", "b"]
arr2 = ["c","b","a"]
arr1 << arr2
puts arr1
puts "#{arr1.length}"
As expected, after <<, arr1 = ["a","b","c","b","a"].
Why arr1.length = 3, as if it were a set?

Answer 1

actually, its ["a","b",["c","b","a"]]

Answer 2

You should do arr1 += arr2

Answer 3

do something like this
for i in 0...arr2.length
arr1 << arr2[i]
end