Which of the following statements is correct? Select one: a. According to the waterfall principle, cashflows are first distributed to the riskiest tranche in an ABS-CDO first. b. After the GFC, it was found that the AAA-rated tranches in ABS and ABS-CDOs were quite thin. c. The same special purpose vehicle has been used multiple times by banks to transfer assets from their balance sheets. d. An AAA-rated senior tranche in an ABS is safer than an AAA-rated senior tranche in an ABS-CDO.