Assertion (A): If $\underset{3}{\overset{5}{\int }}f\left(x\right)dx=10and\underset{0}{\overset{5}{\int }}f\left(x\right)dx=15,then\underset{0}{\overset{3}{\int }}f\left(x\right)dx=25$

Reason(R): $\underset{a}{\overset{c}{\int }}f\left(x\right)dx=\underset{a}{\overset{b}{\int }}f\left(x\right)dx+\underset{b}{\overset{c}{\int }}f\left(x\right)dxfora\le b\le c.$Select the correct answer 

A is false R is true 
 

A is true, R is true and R is correct explanation for A 
 

A is true, R is false 
 

A is true, R is true R is not correct explanation for A