Select the option that is the passive form of the given sentence.

This book explores the secret of leading a long and meaningful life.

A

The secret of leading a long and meaningful life is explored in this book.

B

The secret of leading a long and meaningful life was being explored by this book.

C

The secret of leading a long and meaningful life have been explored by this book.

D

The secret of leading a long and meaningful life had been explored by this book.