What physical feature which lies between two large bodies of water, is the northern border to the muslim empire? ?

The Caucasus Mountains lay between the Black Sea and the Caspian Sea. By the 639, the Arabs invaded the region of what today is known as Azerbaijan and Dagestan (Russia). The Caucasus served as a border between the Muslim Empire and the Turkic Khazar Empire.

the answer should be:

their party has more members.

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b - those executed came from every social and economic class