10.設等差數列<an>的首項a1=198，公差d=-8。若第k項ak為第一個出現小於零的整數，則k=？(A)25(B)26(C)27(D)282

12.設\( f(x) = 3^x \), \( g(x) = \log_{\frac{1}{3}} x \)，則下列選項何者正確？(A)若x1<x2，則\( 3^{x_1} > 3^{x_2} \)(B)若x1<x2，則\( 3^{x_1} + 3^{x_2} = 3^{x_1+x_2} \)(C)若x1<x2，則\( \log_{\frac{1}{3}}(x_2 - x_1) = \log_{\frac{1}{3}} x_2 - \log_{\frac{1}{3}} x_1 \)(D)若x1<x2，則\( \frac{\log_{\frac{1}{3}} x_2 - \log_{\frac{1}{3}} x_1}{x_2 - x_1} < 0 \)