Query values from each json in a json array on postgresql

Question

Each record in my database has a json list, from which I'm trying to return a value inside each element.

I'm trying to return

environmental services, energy & utilities

From

[
  {
    "key": "environmental services"
  },
  {
    "key": "energy & utilities"
  }
]

And.

construction

From

[
  {
    "key": "construction"
  }
]

Where the length of each json list in each record is undetermined.

How can I do this in postgresql?