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# What Is A Group?

TOPO-E5PKXB

Which of the following is a group?

A

$(\mathbb Z/4\mathbb Z-\{\bar{0}\},\times)$

B

$(\mathbb Z/7\mathbb Z-\{\bar{0}\},\times)$

C

$(\mathbb Z,-)$

D

$(\mathbb Q,\heartsuit)\text{ where }a\star b:=\cfrac{a+b}{2}$

E

$(\mathbb R,\star)\text{ where }a\star b:=\sqrt[3]{ab}$