Inherent Limitation of internal control

Dear Gromit,
1- I have a trouble with this question and need you to help me to clarify the inherent limitation of internal control

“The internal auditor recognizes that certain limitations are inherent in any system of internal controls. Which one of the scenarios is the result of an inherent limitation of internal control?

A- The contrompller both makes and records cash deposits
B- The security guard allows one of the warehouse employees to remove assets from the premises without authorization
C- The organization sells to customers on account, without credit approval
D- An employee who is unable to read is assigned custody of the organization’s computer tape library and run manuals that are used during the third shift”

2- Regarding to some materials of internal control, the phrase “avoidable control weaknesses” is used, what is avoidable control weakness?

Thanks a lot