Why is the relationship between the before$-$ and after$-$tax internal rates of return an approximation?

Question $9$ options:

Because the before$-$tax internal rate of return is usually calculated through the process of trial and error

Because in reality expenses offset receipts in the year they occur

Because the before$-$ and after$-$tax internal rates of return are often chosen independently

Because calculation of the present worth of tax savings due to CCA depends on the CTF

Because in reality capital expenses are spread out over time