Here's the tricky thing about verb tenses: sometimes several possible verb tenses could be correct in the same context. Consider:
New information leads me to believe that we were wrong. Present tense - just fine. It's what I'm believing now.
New information lead me to believe that we were wrong. Past tense (because "lead" is one of those weird verbs who past tense looks the same as present tense, but is pronounced differently. English is weird!) - also totally fine! I changed my belief in the past.
New information has lead me to believe that we were wrong. Present perfect - also totally fine! I changed my belief very recently, or am still in the process of doing so.
In this case, don't choose the verb tense that you think it should be, because the present could be right, but so could the present perfect. In fact, although all 3 examples above are perfectly correct, the 3rd one (present perfect) is the one you'll see most often used when we're talking about something that happened in the recent past (in your example, "recent studies") that have ongoing effects in the present (what the doctors conclude). That's what the present perfect is designed to indicate.
That alone doesn't make D or E wrong, though. In D, "lead to doctors concluding" is not idiomatically correct. "LEAD TO" should take a noun, so "lead to a conclusion" is better than "lead to concluding."
E is tricky, because it comes down to a subtlety in meaning. If we say "the doctors," we assume that we mean the same doctors who were mentioned before - the 19th century doctors. This doesn't make sense - they'd be 200 years old! In fact, that's the same issue that makes "them" and "their" wrong in A and B. We can't indicate that it's the same doctors.
So, that just leaves us with C.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education
