What physical feature marks the eastern extent of Alexander's empire?

A great conqueror, in 13 short years he amassed the largest empire in the entire ancient world — an empire that covered 3,000 miles. And he did this without the benefit of modern technology and weaponry.The Persians had already retreated from Ephesus, so Alexander continued to  Alexander returned northwards, and then east towards Persia.Alexander's vast empire began on the Greek peninsula, where his father Philip of Macedon defeated the Greek poleis and united them under his command before his death. Taking command, Alexander invaded the Persian empire, quickly conquering Ionia, Anatolia, Syria, Palestine, and Egypt, and then expanding eastward across the Iranian plateau all the way to India. Mountain ranges and deserts influenced the direction of Alexander's conquests. In the west, theCaucasus mountains to the north and the Arabian Desert to the south directed his path. In the east, the Hindu Kush and the Himalayas forced him to turn south, along the Indus River. After over ten years and thousands of miles, he returned westward to Persia, where he died of uncertain causes.

Explanation:

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The correct answer is b