TOEFL Grammar - Find the error

ScottsEnglish

Find the error in the underlined words in the following sentence:

The studies' conclusions were set to change our understanding of how stars are created, but the astronomers had difficulty to document their observations and hence their conclusions. 

A. studies' conclusions
B. how stars are created
C. had difficulty to document
D. hence their conclusions

Click here for the correct answer.

The studies' conclusions were set to change our understanding of how stars are created, but the astronomers had difficulty documenting their observations and hence their conclusions.