Returning values from an array of JSONB objects.

Hi, please excuse either my stupidity or naivety regarding this but I'm
a bit confused. Give the following basic table structure :

TABLE Data (
ID INT NOT NULL,
Markers jsonb NOT NULL
);

And the following data:

INSERT INTO Data (ID, Markers) VALUES(1, '[ {"idle": true, "items": 8,
"done": 0}, {"idle": true, "items": 8, "done": 0}]') ;

How can I extract the value of the 'items' key either as two rows and or
a sum of both.

I had thought that :

SELECT Markers->'items' AS Items FROM Data WHERE (ID = X) ;

would have done the job but all I get is an empty column, of course using :

SELECT Markers->0->'items' AS Items FROM Data WHERE (ID = X) ;

Produces a single row but isn't exactly what I want. I have of course
searched but not found a definitive answer, or maybe that should read
'one I can understand'.

I'm using PostgreSQL 9.5 on Linux with pgAdmin III on Mac.

--
Bill