Answer: The most effective way to combine sentences 10 and 11 is writing (C.) "Eventually, the Romans used the umbrella to protect themselves from the rain as well as the sun".
Explanation: Option C. presents the best way of combining sentences 10 and 11 into one sentence because it follows the grammatical rules and expresses these ideas in a clear way. Unlike option D, which resorts to the verb "using" instead of "used", option C. does not include mistakes in terms of verb tense. Moreover, option C. places the subject ("the Romans") and the object ("the umbrella") in a correct position, while A. and B. do not.