  I use recent observational and theoretical studies of type Ia supernovae (SNe
Ia) to further constrain the viable SN Ia scenarios and to argue that there
must be a substantial time delay between the end of the merger of the white
dwarf (WD) with a companion or the end of mass accretion on to the WD and its
terminal explosion. This merger/accretion to explosion delay (MED) is required
to allow the binary system to lead to a more or less spherical explosion and to
prevent a pre-explosion ionizing radiation. Considering these recent results
and the required MED, I conclude that the core degenerate scenario is somewhat
more favorable over the other scenarios, followed by the double degenerate
scenario. Although the single degenerate scenario is viable as well, it is less
likely to account for common (normal) SN Ia. As all scenarios require
substantial MED, the MED has turned from a disadvantage of the core degenerate
scenario to a challenge that theory should overcome. I hope that the
requirement for a MED will stimulate the discussion of the different SN Ia
scenarios and the comparison of the scenarios to each other.
