Mr. Smith is very interested in our plan. I spoke to him on the phone last night.

A Mr. Smith is very interested in our plan to whom I spoke on the phone last night.

B Mr. Smith, who is very interested in our plan, I spoke to on the phone last night.

C Mr. Smith, who I spoke on the phone last night, is very interested in our plan.

D Mr. Smith, to whom I spoke on the phone last night, is very interested in our plan.