Dlaczego trzecia linijka jest zaznaczona na szaro i pisze "Namespace declaration is never used"
Chodzi o:
xmlns:app="http://schemas.android.com/apk/res-auto"
<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/spacebackground"
tools:context="com.example.user.quiz.cosmos1">
</android.support.constraint.ConstraintLayout>
