Which of the following statements is incorrect?

$($A$)$Entity Relationship Diagrams are used to design files

$($B$)$ Systems Overview can be prescribed by Data Flow Diagrams

$($C$)$ Indexed files are faster than sequential files in all situations.

$($D$)$ Phases in a Systems life cycle, are fixed and invariant.

$($E$)$System prototyping helps the designer in communicating to the user, quickly, how the system, wh developed, will look like and get a feedback.