For an integral that is to be evaluated using u-substitution, you are given: u= x 7
3
+7. To which of the following integrals can this substitution be successfully applied? ∫(− x 7
21( x 7
3
+7) 3
)dx b) ∫(− x 8
21( x 7
3
+7) 3
)dx c) ∫(− x 7
21e x 7
3
+7
)dx d) ∫(− ( x 7
3
+7) 3
x 9
21
)dx