15. The maximum value of the objective function $z = x + y$ subject to the linear constraints $\left\{ \begin{array} { l } 2 x + y \leq 3 , \\ x + 2 y \leq 3 , \\ x \geq 0 , \\ y \geq 0 \end{array} \right.$ is A. 1 B. $\frac { 3 } { 2 }$ C. 2 D. 3 16. ``$x = 2 k \pi + \frac { \pi } { 4 } ( k \in \mathbf { Z } )$'' is a condition for ``$\tan x = 1$'' that is A. sufficient but not necessary B. necessary but not sufficient C. necessary and sufficient D. neither sufficient nor necessary
(5 points) If the front view of a geometric solid is a triangle, then this geometric solid could be any of the following $\_\_\_\_$ (fill in the numbers of all possible geometric solids) (1) triangular pyramid (2) quadrangular pyramid (3) triangular prism (4) quadrangular prism (5) cone (6) cylinder.
15. The maximum value of the objective function $z = x + y$ subject to the linear constraints $\left\{ \begin{array} { l } 2 x + y \leq 3 , \\ x + 2 y \leq 3 , \\ x \geq 0 , \\ y \geq 0 \end{array} \right.$ is\\
A. 1\\
B. $\frac { 3 } { 2 }$\\
C. 2\\
D. 3
16. ``$x = 2 k \pi + \frac { \pi } { 4 } ( k \in \mathbf { Z } )$'' is a condition for ``$\tan x = 1$'' that is\\
A. sufficient but not necessary\\
B. necessary but not sufficient\\
C. necessary and sufficient\\
D. neither sufficient nor necessary