void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  pinMode(2, INPUT_PULLUP); //Gred A
  pinMode(3, INPUT_PULLUP); //Gred B
  pinMode(4, INPUT_PULLUP); //Gred C
  pinMode(5, INPUT_PULLUP); //Gred D
  pinMode(6, OUTPUT); //Gred A
  pinMode(7, OUTPUT); //Gred B
  pinMode(8, OUTPUT); //Gred C
  pinMode(9, OUTPUT); //Gred D
}

void loop() {
  // put your main code here, to run repeatedly:

  Serial.println("Start to detect:");
  digitalWrite(6,LOW); //LED1 OFF
  digitalWrite(7,LOW); //LED2 OFF
  digitalWrite(8,LOW); //LED2 OFF
  digitalWrite(9,LOW); //LED2 OFF
  delay(1000);

  //Sensor 1 for Grade A
  while(digitalRead(2)==LOW)
  {
    digitalWrite(6,HIGH); //LED1 ON
    digitalWrite(7,LOW); //LED2 OFF
    digitalWrite(8,LOW); //LED2 OFF
    digitalWrite(9,LOW); //LED2 OFF
    Serial.println("GRED A detected");
    delay(1000);
  }
  //Sensor 2 for Grade B
  while(digitalRead(3)==LOW)
  {
    digitalWrite(6,LOW); //LED1 OFF
    digitalWrite(7,HIGH); //LED2 ON
    digitalWrite(8,LOW); //LED2 OFF
    digitalWrite(9,LOW); //LED2 OFF
    Serial.println("GRED B detected");
    delay(1000);
  }
  //Sensor 3 for Grade C
  while(digitalRead(4)==LOW)
  {
    digitalWrite(6,LOW); //LED1 OFF
    digitalWrite(7,LOW); //LED2 OFF
    digitalWrite(8,HIGH); //LED2 ON
    digitalWrite(9,LOW); //LED2 OFF
    Serial.println("GRED C detected");
    delay(1000);
  }
  //Sensor 4 for Grade D
  while(digitalRead(5)==LOW)
  {
    digitalWrite(6,LOW); //LED1 OFF
    digitalWrite(7,LOW); //LED2 OFF
    digitalWrite(8,LOW); //LED2 OFF
    digitalWrite(9,HIGH); //LED2 ON
    Serial.println("GRED D detected");
    delay(1000);
  }
}