“That” or “which” — the eternal question. Which of these two words should you use to start a clause?
The answer depends on how important what you’re writing is. If the clause is important to the meaning, you use “that”; if it can be omitted without changing the meaning, you use “which.”
Essential clauses take “that”
Essential [...]

The problem: The inneffective introduction:
is not organized by key points
is too long and becomes repetitive, losing the reader’s attention

The solution: The effective introduction:
is organized by a thesis listing the key points
is brief and concise