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In order for a white dwarf (WD) to achieve the Chandrasekhar mass, M_C, andexplode as a Type Ia supernova (SNIa), it must interact with another star,either accreting matter from or merging with it. The failure to identify thetypes of binaries which produce SNeIa is the "progenitor problem". Its solutionis required if we are to utilize the full potential of SNeIa to elucidate basiccosmological and physical principles. In single-degenerate models, a WDaccretes and burns matter at high rates. Nuclear-burning WDs (NBWDs) with massclose to M_C are hot and luminous, potentially detectable as supersoft x-raysources (SSSs). In previous work we showed that > 90-99% of the required numberof progenitors do not appear as SSSs during most of the crucial phase of massincrease. The obvious implication is that double-degenerate (DD) binaries formthe main class of progenitors. We show in this paper, however, that manybinaries that later become DDs must pass through a long-lived NBWD phase duringwhich they are potentially detectable as SSSs. The paucity of SSSs is thereforenot a strong argument in favor of DD models. Those NBWDs that are theprogenitors of DD binaries are likely to appear as symbiotic binaries forintervals > 10^6 years. In fact, symbiotic pre-DDs should be common, whether ornot the WDs eventually produce SNeIa. The key to solving the progenitor problemlies in understanding the appearance of NBWDs. Most do not appear as SSSs mostof the time. We therefore consider the evolution of NBWDs to address thequestion of what their appearance may be and how we can hope to detect them.

The Progenitors of Type Ia Supernovae: II. Are they Double-Degenerate Binaries? The Symbiotic Channel