8．（5分）设 ${ \mathfrak { a } } \in \ ( 0 , \ \frac { \pi } { 2 } )$ $\beta \in ( 0 , \ \frac { \pi } { 2 } )$ ，且 $\tan \alpha = \frac { 1 + \sin \beta } { \cos \beta }$ ，则（ ）A $3 a - \beta = \frac { \pi } { 2 }$ B $3 \alpha + \beta = \frac { \pi } { 2 }$ C $2 \alpha - \beta = \frac { \pi } { 2 }$ D $2 \alpha + \beta = \frac { \pi } { 2 }$
Answer key
C