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【单选题】设二维随机变量 \( (X,Y) \) 具有密度函数， \[\qquad\qquad\qquad f(x,y)=\left\{ {{\begin{array}{ll} {\frac{9y^2}{x},} & {0< y< x,0< x< 1} \\ {0,} & \mbox{其他} \\ \end{array} }} \right. \] 则 \( X ...
A.
\( f_X (x)=\left\{ {{\begin{array}{ll} {x,} & {0< x< 1} \\ {0,} & \mbox{其他} \\ \end{array} }} \right. \)
B.
\( f_X (x)=\left\{ {{\begin{array}{ll} {2x,} & {0< x< 1} \\ {0,} & \mbox{其他} \\ \end{array} }} \right. \)
C.
\( f_X (x)=\left\{ {{\begin{array}{ll} {3x^2,} & {0< x< 1} \\ {0,} & \mbox{其他} \\ \end{array} }} \right. \)
D.
\( f_X (x)=\left\{ {{\begin{array}{ll} {4x^3,} & {0< x< 1} \\ {0,} & \mbox{其他} \\ \end{array} }} \right. \)