Rspec 2 concatenate

The instructions read thusly:

### Concatenate

Create a method that takes in an Array of Strings and uses inject

to return the concatenation of the strings.

```ruby

concatenate(["Yay ", "for ", “strings!”])

# => “Yay for strings!”

```

Before I zip up my folder and submit this project I wanted to make absolutely sure that I am right:
Does the problem intend for me to use the built-in INJECT, or my own MY_INJECT?
With built-in inject all I have to do to make it work is to use (:+) but that seems like it should be too simple.
After al the yielding and block calling in the rest of the project it just seems odd that there would be one so simple at the very end.
Please respond soon

It asks you to use the built-in inject!