In the **parallelogram law** of vector addition, if $\vec a$ and $\vec b$ act along two adjacent sides $\overrightarrow{AB}$ and $\overrightarrow{AD}$ of a parallelogram, then $\vec a + \vec b$ is represented by: 
AThe opposite side $\overrightarrow{DC}$
BThe diagonal $\overrightarrow{AC}$
CThe diagonal $\overrightarrow{BD}$
DThe side $\overrightarrow{CB}$
Answer & Solution
Correct answer: B. The diagonal $\overrightarrow{AC}$
Parallelogram law: with $\overrightarrow{AB} = \vec a$ and $\overrightarrow{AD} = \vec b$, the resultant $\vec a + \vec b$ is the diagonal $\overrightarrow{AC}$ emanating from the common initial point A. (The other diagonal $\overrightarrow{BD}$ represents $\vec b - \vec a$.)
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