BF$_3$ = NH$_4^+$ < NF$_3$ < NH$_3$

BF$_3$ < NH$_4^+$ < NF$_3$ < NH$_3$

NH$_4^+$ < BF$_3$ < NH$_3$ < NF$_3$

BF$_3$ < NH$_4^+$ < NH$_3$ < NF$_3$

P & Q are identical, and R & S are diastereomers.

P & Q are diastereomers, and R & S are identical.

P & Q are diastereomers, and R & S are diastereomers.

P & Q are identical, and R & S are identical.

$E_{2s}({Li}) < E_{2p}({Li})$

$E_{2s}({H}) = E_{2p}({H})$

$E_{2p}({H}) < E_{2s}({Li})$

$E_{2s}({H}) > E_{2s}({Li})$

X is used for sterilizing drinking water.

Y has a planar structure.

Z is used in the enrichment of $^{235}$U.

Y is a stronger Lewis base than ammonia.