问题如下：
Good-on-open-order is a market order. This kind of order will be submitted at the market’s open tomorrow and is filled at the market price. Tim have placed a sell market-on-open order. Which of the following statement about Tim's instruction is correct?
选项：
A.Tim instruction, to sell the shares at the market open, is an execution instruction.
B.Tim instruction, to sell the shares at the market open, is a validity instruction.
C.Tim instruction, to sell the shares at the market open, is a clearing instruction.
解释：
B is correct
A validity instruction specify when an order should be executed.
讲义上面说Good-on-close-order often 是market order。请问good-on-close/open order是不是通常默认为是market order？
