Answer: The most effective way to combine sentences 10 and 11 is writing: "Eventually, the Romans used the umbrella to protect themselves from the rain as well as the sun".
Explanation: The best way to combine sentences 10 and 11 is keeping the structure of sentence 10 and then adding the phrase "as well as" in order to include "the sun". The last option ("Eventually, the Romans used the umbrella to protect themselves from the rain as well as the sun") represents the most effective way of joining both sentences into one because it is clear and, most importantly, it is a grammatical sentence. In contrast, 1) separates the subject from the rest of the elements in the sentence, 3) separates the direct object from the other elements, and 2) uses Present Continuous Tense when the writer is actually making reference to events and customs from the past.