B, does not COnform to around 8: 00 a.m., 1 1: 00, 2: 00 p.m. and 6: 00 p.m., and the co content is high and rises at night, so b is wrong;
C, there is no obvious change in each time period, so it is inconsistent with the actual situation, so C is wrong;
D at 8 am, around 1 1 pm, at 2 pm and 6 pm, the CO content is higher, but less at night, which is consistent with the reality, so d is correct.
Therefore, choose: d.