In the US, people can return to school in their late 20s, 30s, or older to get a higher degree such as a Master or Ph.D. like Rachel, many more are taking training course to (40) their skills
In the US, people can return to school in their late 20s, 30s, or older to get a higher degree such as a Master or Ph.D. like Rachel, many more are taking training course to (40) ___ their skills.
A. improve
B. higher
C. rise
D. raise