Answer:
The most effective way to combine sentences 10 and 11 is 1. Eventually the Romans used the umbrella to protect themselves from the rain as well as the sun.
Step-by-step explanation:
In the question, numbers 11 and 12 have almost the same sentences except for the ending parts. So, instead of writing the same words twice, merging the two sentences and altering them into "from the rain as well as the sun" makes the sentence more easier to read as well as understand. So, the final conclusion about how the sentence should be is option 1. Eventually the Romans used the umbrella to protect themselves from the rain as well as the sun.