get the first (or n'th) element in a jq json parsing

I can get the 1st element in a JSON inside []

$ echo \'[{"a":"x", "b":true}, {"a":"XML", "b":false}]' | \    jq '.[1]'

{"a":"XML","b":    false}

But if the json is already disassembled (for instance, after filtering entries using 'select'), how can I choose a single entry and avoid the error seen here?

$ echo \'[{"a":"x", "b":true}, {"a":"x", "b":false}, {"a":"XML", "b":false}]' | \    jq \'.[] | select( .a == "x")'

{"a":"x","b":    true}{"a":"x","b":    false}

$ echo \'[{"a":"x", "b":true}, {"a":"x", "b":false}, {"a":"XML", "b":false}]' | \    jq \'.[] | select( .a == "x") | .[1]'

jq: error (at <stdin>:1): Cannot index object with number