Receptors found in signal transduction
a. are always found in membranes.
b. are always found in the cytoplasm.
c. are always found in the nucleus.
d. can be found either in the membrane, cytoplasm, or nucleus.

answer: land bridge theory. land bridge theory held that shifting portions of dry land explained the finding of identical species on different continents. plate tectonic theory eventually eliminated the land bridge theory.