问题如下：
Which of the following statements not one of the flaws in the securitization process prior to the start of the credit crisis in 2007?
选项：
A. An active originate-to-distribute model where a strong profit motive took precedence over ethical lending and underwriting.
B. The securitized products were so opaque that investors could not evaluate the true risks of the investment.
C. Structured investment vehicles (SIVs) were used to enhance the risk discovery process for investors and regulators.
D. Banks held securitized assets in off-balance-sheet entities, thus further masking the true risks in the system.
解释：
C Structured investment vehicles (SIVs) were actually used to create further layers of opaqueness. These are the off-balance-sheet entities used by banks to hold securitized products in a way that made them very difficult for investors to scrutinize.
请问，选项D是什么意思？