The HC-SR04 ultrasonic sensor is a very affordable proximity/distance sensor that has been used mainly for object avoidance in various robotics projects. It essentially gives your Arduino eyes/spatial awareness and can prevent your robot crashing against a wall. It has also been used in turret applications, water level sensing, and even as a parking sensor.
Here you can find my answers to the exercises and practice proposed at page Arduino [Unit 1.1] - Blink. You should read that article and try to solve the exercises and practical proposals in the laboratory before reading the answers on this page.
Start up the Arduino software IDE (Integrated Development Environment) and open the Blink example sketch (reported below) that is available in the File>Examples>Analog menu of the Arduino/Genuino software. Sketches are written in Arduino C.