Which of the following are correct about boron trihalides?

AThe $B - F$ bond in $BF X_{3}$ is shorter than a pure single bond owing to $p π$ - $p π$ back donation.
B $BF X_{3}$ is a stronger Lewis acid than $BCl X_{3}$ because fluorine is the most electronegative element, so it withdraws the most electron density from boron.
C $BBr X_{3}$ is a stronger Lewis acid than $BF X_{3}$ .
D $BF X_{3}$ forms the adduct $F X_{3} B \cdot NH X_{3}$ in which boron becomes $s p^{3}$ hybridised.

Answer: A, C, D

Back donation shortens

B - F

bonds and reduces the acidity of

BF X_{3}

. Adduct formation uses the vacant

2 p

orbital, giving

s p^{3}

boron. As back donation weakens down the group,

BBr X_{3}

is the strongest acid. The reverse claim in the last option is false.