This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cranial cavity, Subject name: Anatomy
Which of the following cavities are separated from the middle cranial fossa by a thin layer of bone? 
Select and explain. 
A. Auditory tube and bony orbit, B. Middle ear cavity and sphenoid sinus, C. Sigmoid sinus and frontal sinus, D. Sphenoid sinus and ethmoid sinus
B: Middle ear cavity and sphenoid sinus
Answer: B: Middle ear cavity and sphenoid sinus 
Explanation: The middle ear cavity is separated from the middle cranial fossa by the tegmen tympani, a thin plate of the petrous pa of the temporal bone. A pa of the roof of the sphenoid bone forms the floor of the hypophyseal fossa. The other pairs of sinuses or bony cavities are not separated from the middle cranial cavity.