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For Mac OS users, there is a system setting that may not allow you to tab onto several types of elements in a web page. To change this setting:
1st Block: https://classroom.google.com/c/NTM4MzQ1MjQ2NjMz
2nd Block: https://classroom.google.com/c/NTM4MzQ2NTM4NzIy
3rd Block: https://classroom.google.com/c/NTM4MzQ4NTYwMTU4
5th Block: https://classroom.google.com/c/NTM4MzUwODI5NTY2
6th Block: https://classroom.google.com/c/NTM4MzUxMTY1MTc2
7th Block: https://classroom.google.com/c/NTM4MzUwOTIxNDk0