Which structure prevents blood flowing back into the heart from the aorta between heartbeats?

A. semilunar valve
B. atrioventricular valve
C. tricuspid valve
D. mitral valve
E. ductus arteriosus


A. semilunar valve

Biology & Microbiology

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Two bird species have different mating calls, occupy different habitats in nature, and when they hybridize, the hybrids are infertile. Which isolating mechanisms maintain these species?  

A.  ecological isolation, behavioral isolation, and hybrid sterility B.  mechanical isolation, hybrid sterility, geographical isolation C.  ecological isolation, mechanical isolation, hybrid sterility D.  geographical isolation, temporal isolation, behavioral isolation Clarify Question · What is the key concept addressed by the question? · What type of thinking is required?   Gather Content · What do you already know about reproductive isolation?   Choose Answer · Given what you now know, what information and/or problem solving approach is most likely to produce the correct answer?   Reflect on Process · Did your problem-solving process lead you to the correct answer? If not, where did the process break down or lead you astray? How can you revise your approach to produce a more desirable result?

Biology & Microbiology

Signals from motor neurons result in action potentials in muscle fibers, which in turn causes the __________ to

release stored calcium ions. Fill in the blank(s) with the appropriate word(s).

Biology & Microbiology

During which stage(s) of cellular respiration is CO2 produced?

A. Krebs cycle and electron transport chain B. oxidation of pyruvate to acetyl-CoA and Krebs cycle C. glycolysis and Krebs cycle D. Krebs cycle only E. electron transport chain

Biology & Microbiology

Plants compete for light, a response known as ____, in which plants tend to grow taller when closely surrounded by

other plants. a. epinasty b. phototropism c. shade avoidance d. shade tolerance e. thigmotropism

Biology & Microbiology