Yeah, I saw it a couple slides back with a picture but it was one of the wrong answers on that one. Guess the moral is always check the other answers even if you immediately know the correct one. Also, mine had both all 3 characters on one and the first character on one and the last two on another. Don't know if it matters which you pick as long as you get all the characters in the correct order.
Dont forget you can tap on the word and if will tell you what it is. So if a japanese word comes up youve never seen, just tap it, and it will tell u what it means. Which is helpful for when this happens. I think it happens because the things in each lesson are randomized. So i got the four pictures first before i was asked what せなか meant without a picture. But other times i havent. And ive got the word before being told what it even means, which is when being able to tap it comes in handy. It works the other way too. Say the word 'apple' came up but you hadnt been taught it yet. You just tap the word in the question and will come up with a little box showing you what apple is in japanese like りんぐ- apple
Both are sets of characters for writing syllables. There is the same number of characters in each system, 48, and they are both in the same syllabic order:<pre>
A-I-U-E-O, KA-KI-KU-KE-KO, SA-SHI-SU-SE-SO, ....</pre>
The hiragana are rounded, while the katakana are angular. Some of the syllabic characters look alike in both sets, for example RI, .... but not all.
I found hiragana immediately useful for reading train station names from a distance, and katakana for reading menus.
Both of them. They are not interchangeable. If there are not any loan words in a text you only see hiragana along with kanji. If there are loan words you see katakana with hiragana and kanji. And it is important to know that there are always some hiragana in a sentece along with kanji.
Kanji, Hiragana, and Katakana are explained in other comments, in wikipedia, and the first 4 google/msn/duckduckgo search results. The "blocks"people are referring to are the words or pieces of words that you can select to build a correct answer for practice questions that use them.