Assertions on JUnit test throw errors

Question

I am just starting my journey in the programming language of Java and I just cannot understand why I got these two errors, when I try to do a Junit test:

Access restriction: The method 'Assert.assertEquals(Object, Object)' is not API (restriction on required library 'C:\eclipse\plugins\org.junit_4.12.0.v201504281640\junit.jar') ConcateTest.java    /TestProject/src/test   line 13 Java Problem
Access restriction: The type 'Test' is not API (restriction on required library 'C:\eclipse\plugins\org.junit_4.12.0.v201504281640\junit.jar')  ConcateTest.java    /TestProject/src/test   line 5  Java Problem

If you wonder what I have written so far I just leave it here:

package test;

import static org.junit.Assert.*;

import org.junit.Test;

public class ConcateTest {


    public void testConcatenate() {
        Junit test = new Junit();
        String result= test.Concatenate("one","two");
        assertEquals("onetwo",result);
    }
}

And this is how my Junit class looks like: package test;

public class Junit {
    public String Concatenate(String one,String two)
    {
        return one+two;
    }

    public int Multiply(int number1,int number2)
    {
        return number1*number2;
    }
}

I would very much appreciate it if somebody would tell me how I can fix this problem, thanks in advance!