Complex sentence: The County class were the first and last heavy cruisers of the Royal Navy, being built in four sub-classes, that became known as the "Kent", "London", "Norfolk" and "York" classes.
Split into simpler sentences: The County class were the first and last heavy cruisers of the Royal Navy. The "County class" were built in four sub-classes, that became known as the "Kent", "London", "Norfolk" and "York" classes.