What is the best paraphrase for the following sentence from the Web article above?

Over the past 22 years, scientists have performed more than 30 clinical studies in an attempt to determine how effective St. John's wort is.
A. To see if St. John's wort is really helpful, more than 30 studies have been conducted in the past 22 years (Tocco 2011).
B. According to scientists around the world, St. John's wort may or may not be helpful (Tocco 2011).
C. Unfortunately, scientists continue to find it impossible to determine the effectiveness of St. John's wort (Tocco 2011).

Question

What is the best paraphrase for the following sentence from the Web article above?

Over the past 22 years, scientists have performed more than 30 clinical studies in an attempt to determine how effective St. John's wort is.
	A. 	To see if St. John's wort is really helpful, more than 30 studies have been conducted in the past 22 years (Tocco 2011).
	B. 	According to scientists around the world, St. John's wort may or may not be helpful (Tocco 2011).
	C. 	Unfortunately, scientists continue to find it impossible to determine the effectiveness of St. John's wort (Tocco 2011).

anonymous · Answer

D. 	Scientists have performed more than 30 clinical studies to test St. John's wort (Tocco 2011).

anonymous · Answer

I think it's either A or D