Why can I use querySelector to select class, on question 3, but not on number 5?

// 1: Select the element with the ID 'about'. 
//    Store the element in the variable `about`.


const about = document.querySelector("#about");
about.style.border = "2px solid firebrick";

// 2: Select all the <h2> elements in the document.
//    Set the color of the <h2> elements to a different color.

const h2Tag = document.getElementsByTagName("h2");
for (var i = 0; i < h2Tag.length; i++) {

h2Tag[i].style.color = "green";

}


const card = document.querySelector(".card");
card.style.backgroundColor = "yellow";


// 3: Select all elements with the class '.card'. 
//    Set their background color to the color of your choice.



// 4: Select only the first <ul> in the document.
//    Assign it to a variable named `ul`.
const ul = document.querySelector("ul");



ul.style.border = "2px solid indigo";

// 5: Select only the second element with the class '.container'.
//    Assign it to a variable named `container`.

const container = document.getElementsByClassName("container")[1];

container.style.backgroundColor = "royalblue";

// 6: Select all <a> elements that have a 'title' attribute. 
//    Set their color value to the color of your choice.