Because / Because of / Due to

We delayed our trip ​ (a)   Dad was sick with the flu.

• because
• because of

Claire's eyes were red ​ (a)   she had been crying.

• because
• due to

The water in most rivers is unsafe to drink ​ (a)   pollution.

• because of
• because

The water in most rivers is unsafe to drink ​ (a)   it is polluted.

• because
• because of

Some people think Harry succeeded in business ​ (a)   his charming personality rather than his business skills.

• because of
• because

You cannot enter this secured area ​ (a)   you don't have proper ID.

• because
• due to

My lecture notes were incomplete ​ (a)   the instructor talked too fast.

• because
• because of
• due to

It is unsafe to travel in that country ​ (a)   the ongoing civil war.

• due to
• because