4. The_layer of the GI tract wall contains glands, blood vessels and nerves, whereas the__ layer is the outermost layer that protects the GI tract.
(A) mucosa, muscle
(B) serosa, submucosa
(C) submucosa, muscle
(D) submucosa, serosa

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統計： A(2), B(2), C(1), D(0), E(0) #1761462