JSFiddle - React, Tailwind, and code Playground

HTML

<label for="select_class">Class:
    <select id="select_category" class="selectSome">
		<option value="null">Please select...</option>
        <option value="c1">Class 1</option>
        <option value="c2">Class 2</option>
        <option value="c3">Class 3</option>
    </select>
</label><br/>
<label for="select_subject">Subject: 
    <select id="select_subject" class="selectSome">
		<option value="null">Please select...</option>
        <option value="s1">Subject 1</option>
        <option value="s2">Subject 2</option>
        <option value="s3">Subject 3</option>
    </select>
</label>
<hr/>
<div class="row" id="row_s1_c1">
Subject 1 for Class 1
</div>
<div class="row" id="row_s2_c1">
Subject 2 for Class 1
</div>
<div class="row" id="row_s1_c2">
Subject 1 for Class 2
</div>
<div class="row" id="row_s2_c2">
Subject 2 for Class 2
</div>
<div class="row" id="row_s3_c2">
Subject 3 for Class 2
</div>
<div class="row" id="row_s3_c3">
Subject 3 for Class 3
</div>

CSS

.row {
    display: none;
}

JavaScript

$().ready(function() {
    $('.selectSome').on('change', function() {
        showHide();
    });
});

function showHide() {
    // hide all rows
    $('.row').hide();
    // show good row only
    if ($('#select_category').val() != 'null' && $('#select_subject').val() != 'null') {
        $('#row_' + $('#select_subject').val() + '_' + $('#select_category').val()).show();
    }
}