Select the correct words among the given options to fill the blanks below the sentence:

The great wall of china was build __________ to keep invaders out.
Options: a) strongly, b) very strongly, c) powerfully, d) formidably
The great wall of china was built formidably to keep invaders out.
Complete the following task based on the instruction and optional input provided.### Instruction:
Select the correct words among the given options to fill the blanks below the sentence:

The great wall of china was build __________ to keep invaders out.

### Input:
Options: a) strongly, b) very strongly, c) powerfully, d) formidably

### Output:
The great wall of china was built formidably to keep invaders out.