Which pair of molecules are polar species?

$BF_3$ and $NF_3$ both are covalent compounds but $NF_3$ is polar whereas $BF_3$ is non-polar. This is because

The correct order of dipole moment for the following molecules is:

Statement $I$: Dipole moment is a vector quantity and by convention it is depicted by a small arrow with tail on the positive centre and head pointing towards the negative centre.
Statement $II$: The crossed arrow of the dipole moment symbolizes the direction of the shift of charges in the molecules.
In the light of the above statements,choose the most appropriate answer from the options given below :-

Which of the following compounds is polar?

Explain why $BeH_{2}$ molecule has a zero dipole moment although the $Be-H$ bonds are polar.