Twins Cheri and Keri often speak in code that other people are unable to understand. Because it is perfectly normal to them to communicate in that manner, they may be unaware that they are using .

Sign language? or morse code

Explanation:

the correct answer is the following is a list of inventions made in the medieval islamic world, especially during the "islamic golden age" (8th to 13th centuries),[1][2] as well as the late medieval period, especially in the emirate of granada and the ottoman empire.

science and technology in the islamic world adopted and preserved knowledge and technologies from contemporary and earlier civilizations, including persia, india, china, and greece-roman antiquity, while making a number of improvements and innovations.