The not knowing if he was or he was n't . It was literally killing me ! Fortunately , a friend recommended a book to me that changed my life .

Based on the preceding passage, choose your answer for question Why is the narrator so grateful to the friend ?
OPTIONS:
A). The friend has recommended them for a course .;
B). The friend has recommended them for a loan .;
C). The friend has recommended them a job .;
D). The friend has recommended them a great book .;
The answer is:
D).