PostgreSQL jsonb Quiz: Containment, Existence and JSON Null

PostgreSQL jsonb Quiz: Containment, Existence and JSON Null

Choose one answer. Each attempt draws 10 of 20 questions; the practice pass mark is 8/10. Original topic practice, not an official exam. Read jsonb operators without confusing nested keys, strings, arrays and SQL NULL.
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What does this PostgreSQL query return?
sql
SELECT ('{}'::jsonb -> 'a') IS NULL;