Which of the following statements is true? a. One expert cannot testify about the opinions of another expert. b. An expert's opinion at trial may only be based on the evidence that has already been presented. c. Blue, an attorney, plans to call Green, a CFE, as an expert witness at trial. A week before the trial, the two meet to discuss their strategy. Blue tells Green what questions he will ask at trial and the two discuss how Green's evidence should be presented to the jury. Blue is subject to sanction for witness tampering. d. Substantive exhibits used by experts in conjunction with testimony do not have to be prepared by the expert, but the expert must supervise the preparation and must be able to verify all calculations and underlying data.