The airplane manufacturer realized a flaw in its design when tests revealed that the cabin door required toomuch force to be opened in an emergency: for safety purposes, the door needs to be difficult to open butnot ______.A. indelibleB. immutableC. immobileD. impossible

To answer the question, we need to find a word that fits the context of the sentence. Let's break it down step by step:

1. Understand the context: The sentence discusses a flaw in the design of an airplane cabin door. The door requires too much force to be opened in an emergency. For safety purposes, the door needs to be difficult to open but not something.

2. Identify the requirement: The door should be difficult to open but not impossible to open. This means the word we are looking for should convey the idea that the door should not be so difficult to open that it cannot be opened at all.

3. Analyze the options:

   • A. indelible: This means something that cannot be removed or erased. This does not fit the context of the door.
   • B. immutable: This means something that cannot be changed. This also does not fit the context of the door.
   • C. immobile: This means something that cannot be moved. This could fit the context, but it is not the best choice.
   • D. impossible: This means something that cannot be done. This fits the context perfectly because the door should not be impossible to open.

4. Choose the best option: Based on the analysis, the best word to complete the sentence is "impossible."

So, the completed sentence is: "The airplane manufacturer realized a flaw in its design when tests revealed that the cabin door required too much force to be opened in an emergency: for safety purposes, the door needs to be difficult to open but not impossible."