7136/3 Economics Jun 2023 Quiz | AQA A Level MCQ

The diagram below shows the demand and supply curves for skilled labour (D$_s$ and S$_s$) and unskilled labour (D$_u$ and S$_u$) and the wage differential (W$_s$–W$_u$) between the two types of labour.
All other things being equal, which one of the following is most likely to cause a decrease in the wage differential?