1. What do the following letter pairs have in common?

DO
FO
HO
PO
SO
TO

2. What do the following letter groups have in common?

CRA
DRI
HEA
LEA
PRO

Answers:

1. Each letter pair can be made into a word by adding UR:

DOUR
FOUR
HOUR
POUR
SOUR
TOUR

2. Each letter group can be make into a word by adding VEN:

CRAVEN
DRIVEN
HEAVEN
LEAVEN
PROVEN