How long is the section bordering the northern frontier great wall?

Correct answer is: The section bordering the northern frontier is about 70 kilometers long. The Great Wall is a Chinese fortification built in order to protect the northern border of the Chinese Empire, and it’s considered one of the Wonders of the World. On average, it measures 6 to 7 meters high and 4 to 5 meters wide, and is estimated to be about 21,200 kilometers long.  

offline recommendation systems option b

because recommendation systems produce a list of two ways

1. through collaborative filtering

or

2. through content-based filtering