No matter where you put 5 in box 1 the cell C7 cannot contain the value 5.
If the value 5 is in box 1 then E3
can't be 5 (The value can appear only once in the box),
so E7 must be 5 (The value must appear in the col),
so C7 can not be 5 (The value can appear only once in the box).
If the value 5 is in col C then C7 can't be 5 (The value can appear only once in the col)
Hence there are only two possible configurations, neither of which allow
C7 to be 5,
Therefore we can remove C7-5.