for t in range(int(input())):
k = 0
ans = ""
S = set()
a = input()
for i in a:
if i == " ":
continue
cnt = a.count(i)
if k<cnt:
S.clear()
S.add(i)
k = cnt
ans = i
elif k == cnt and ans != i:
S.add(i)
if len(S) > 1:
print("?")
else:
print(ans)