199. Initially, the number of radioactive nuclei of elements A and B are equal. After time $\Delta t$, the remaining nuclei of element A are four times the remaining nuclei of element B. If the half-life of element B is equal to the half-life of element A, and $n_A$ and $n_B$ are the number of remaining nuclei of A and B after time $\Delta t$, which of the following is correct? (1) $n_A - n_B = 4$ (2) $n_B - n_A = 4$ (3) $n_A - n_B = 2$ (4) $n_B - n_A = 2$
\textbf{199.} Initially, the number of radioactive nuclei of elements A and B are equal. After time $\Delta t$, the remaining nuclei of element A are four times the remaining nuclei of element B. If the half-life of element B is equal to the half-life of element A, and $n_A$ and $n_B$ are the number of remaining nuclei of A and B after time $\Delta t$, which of the following is correct?
\hspace{0.5cm}(1) $n_A - n_B = 4$ \hspace{1cm} (2) $n_B - n_A = 4$ \hspace{1cm} (3) $n_A - n_B = 2$ \hspace{1cm} (4) $n_B - n_A = 2$