When a researcher is selecting items for a composite index, which of the following should be kept in ...

When a researcher is selecting items for a composite index, which of the following should be kept in mind?
 
  a. Each item should have face validity.
  b. A composite measure should be unidimensional.
  c. There should be concern about the amount of variance provided by the items.
  d. Each item should have face validity and the composite measure should be unidimensional.
  e. Each item should have face validity, the composite measure should be unidimensional, and there should be concern about the amount of variance provided by the items.