The answer choices are:
a. complex stability is lost if the complexes are able to interact structurally
b. complexes have different cellular locations, and it is not feasible to isolate them together
c. the complexes all use the same substrates, so their use must be monitored separately
d. the reactions catalyzed by the complexes are coupled to one another
Best answer choice is:
d. the reactions catalyzed by the complexes are coupled to one another
Studying the complexes together would lead to errors because inhibition of Complex 1 and 2 will affect the activity of Complex 2, and therefore it will affect the activity of complex 4.