Which of the following molecules contain polar covalent bonds but is a nonpolar molecule?

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Polar bonds exist when bonds form between different atoms. All of the molecules except for $\ce{N2}$ contain polar bonds.

$\ce{CCl4}$ is a tetrahedral molecule. Since all four bonds are the same, symmetry exists and the molecule is nonpolar.

$\ce{NH3}$ is polar since the lone pair electrons result in a trigonal pyramidal structure instead of a symmetrical tetrahedral.

$\ce{CH2Cl2}$ is polar because the two chlorine and carbon bonds form a region of higher electron density than the bonds between hydrogen and carbon. There will be a net dipole moment due to this.