Extract the answer to the question from the following context. Question: What was the cause of the Buddhist religion's influence from other cultures? Context: The gradual spread of Buddhism into adjacent areas meant that it came into contact with new ethnical groups. During this period Buddhism was exposed to a variety of influences, from Persian and Greek civilization, to changing trends in non-Buddhist Indian religions—themselves influenced by Buddhism. Striking examples of this syncretistic development can be seen in the emergence of Greek-speaking Buddhist monarchs in the Indo-Greek Kingdom, and in the development of the Greco-Buddhist art of Gandhāra. A Greek king, Menander, has even been immortalized in the Buddhist canon.
The answer to this question is:
The gradual spread of Buddhism into adjacent areas