Because according to the provisions of Shanghai Futures Exchange on warehouse receipts and securities offsetting trading deposits, registered warehouse receipts and securities offsetting deposits are calculated according to the following rules:

1, not exceeding 80% of the total value of registered warehouse receipts and securities (calculated at the settlement price of the previous day).

2. The maximum amount shall not exceed 4 times the actual monetary deposit of the member.

So 40 million national debt is 32 million according to 80% of the pledged amount, but because there are actually only 7 million monetary deposits, four times is 28 million, so 40 million national debt can only pledge 28 million deposits, so choose answer C.

As for the topic of futures calculation, the key is to have a deep understanding of the relevant trading and settlement rules. Like the above topics, it is very simple for those who understand the rules of the exchange.