Mammalian circulatory system features

Steps:

• Recall that circulation types are classified as open (blood flows freely in body cavity) or closed (blood stays in vessels).
• Identify mammalian blood flow: heart pumps blood to lungs (pulmonary) then back to heart, and to body (systemic) then back.
• Confirm double circulation means two separate loops: pulmonary and systemic.
• Conclude mammals use closed vessels for efficient oxygen delivery.

Why B is correct:

• Mammals have closed circulation (blood confined to vessels for high pressure) and double circulation (separate pulmonary and systemic circuits, as defined in vertebrate physiology).

Why the others are wrong:

• A: Mammals do not have open circulation; that's in insects and some mollusks.
• C: Mammals lack single circulation; fish have a single loop without separation.
• D: Open circulation is absent in mammals, and single is for simpler organisms.

Final answer: B