Who were the main actors of the trans Saharan trade

Slave and Ivory

Explanation:

The trans Saharan trade wouldn't continue if they were available

Correct answer choice is:

C. A stopwatch

Explanation:

The best a decent eye will do is fix a reading to one tenth of a second, that is so the design of the stopwatch. The digital stopwatch has 2 digits on the far side the seconds, thus it further separates the time in hundredths of a second. Since it's simple to scan to hundredth of a second, that's its design.