On Challenge Task 5 of 6(Intro to Git), what is the option that will show the staged changes?

On Challenge Task 5 of 6(Intro to Git), I'm trying to find the option that will show the staged changes. Here's the question (below):

Run the "git diff" command again, but this time add the option that will show you the staged changes. (When you have the command right, changes to both the file that was previously tracked and the file that was previously untracked will be shown.)

I tried a couple of options. For example, I tried "git diff -- staged", but that wasn't right.

I also tried "git diff chapter1.txt", but that wasn't right either. Here's the error message I received:

Type in your command below, then press Enter. Bummer: All the files are staged, so "git diff" by itself won't produce any output! There's a command-line option you need to add to the "git diff" command..

I tried to find the command line option that will work, but I'm coming up blank. Any ideas?

Thanks,

Jan