smc

By the rules of exponents, $10^{-49}=10\ast 10^{-50}$, which clearly exceeds $2\ast 10^{-50}$. $10^{-49}-2\ast 10^{-50}=10\ast 10^{-50}-2\ast 10^{-50}=8\ast 10^{-50}$, so we choose answer choice $\boxed{\text{the first exceeds the second by }8\cdot 10^{-50}}$.