Answer:
The most effective way of combining sentence 10 and 11 is by using as well as.
EXPLANATION:
In the passage given above, the writer is trying to trace the development of the discovery of umbrella and its different usage.
All the information about the discovery and usage of umbrella from ancient Mesopotamia to ancient Roman empire.
In the sentence 10 and 11 there is an explanation given that why umbrellas were used.
Since, the usage of umbrellas were for two reasons, for protecting ourselves from sun and rain, so both of the sentences could be connected together by as well as.
As well as is used as a conjunction and it is often used in place of and for combining two sentences.
Explanation:
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