A study group at a school conducted seed germination experiments at 20 different temperature conditions to investigate the relationship between the germination rate $y$ of a certain crop seed and temperature $x$ (in ${}^{\circ}\mathrm{C}$). From the experimental data $\left( x _ { i } , y _ { i } \right) ( i = 1,2 , \cdots , 20 )$, a scatter plot was obtained. Based on this scatter plot, between $10^{\circ}\mathrm{C}$ and $40^{\circ}\mathrm{C}$, which of the following four regression equation types is most suitable as the regression equation type for the germination rate $y$ and temperature $x$? A. $y = a + b x$ B. $y = a + b x ^ { 2 }$ C. $y = a + b \mathrm { e } ^ { x }$ D. $y = a + b \ln x$
A study group at a school conducted seed germination experiments at 20 different temperature conditions to investigate the relationship between the germination rate $y$ of a certain crop seed and temperature $x$ (in ${}^{\circ}\mathrm{C}$). From the experimental data $\left( x _ { i } , y _ { i } \right) ( i = 1,2 , \cdots , 20 )$, a scatter plot was obtained. Based on this scatter plot, between $10^{\circ}\mathrm{C}$ and $40^{\circ}\mathrm{C}$, which of the following four regression equation types is most suitable as the regression equation type for the germination rate $y$ and temperature $x$?
A. $y = a + b x$
B. $y = a + b x ^ { 2 }$
C. $y = a + b \mathrm { e } ^ { x }$
D. $y = a + b \ln x$