Final answer:
Santiago is surprised by the lack of any water source in the Al-Fayoum oasis.
Step-by-step explanation:
d. The lack of any water source
Santiago is surprised by the lack of any water source in the Al-Fayoum oasis. This is unexpected because oases are typically formed around underground water sources that reach the surface via wells or springs, providing a vital water supply for agriculture and habitation in desert regions. However, in the case of the Al-Fayoum oasis, there is no natural water source, making it unique and surprising.