This model represents the expression .
n
+
n
+
d
+
d
+
d
+
d
n
n
d
d
d
d
Which expression is equivalent to ?
n
+
n
+
d
+
d
+
d
+
d
6
n
d
3
n
+
4
d
2
n
+
4
d
3
n
+
3
d
ref_doc_title.
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