Cho 2 vectơ 

Câu hỏi: Cho 2 vectơ $\overrightarrow{\mathrm{a}}; \overrightarrow{\mathrm{b}}$ có $\left|\overrightarrow{\mathrm{a}}\right|$=4; $\left|\overrightarrow{\mathrm{b}}\right|$=5; ($\overrightarrow{\mathrm{a}}; \overrightarrow{\mathrm{b}}$)=${120}^0$. Tính: $\left|\overrightarrow{\mathrm{a}}+\overrightarrow{\mathrm{b}}\right|$

A. $\sqrt{21}$

B. $\sqrt{61}$

C. 21

D. 61