The cats have full bowls in the morning and afternoon, Katy can assume that the cats do not eat in the morning or afternoon. The bowls are replenished in the evening, which suggests that they become empty in the evening, which suggest that the pattern is that the cats eat in the evening so your answer would be C. Hope this helps. ;)
Electricidade is electricity in English
Assuming our "closed tube" is closed at only one end, then
<span> v = fλ = f*4L/n
</span><span> where "n" is the harmonic number. So
</span><span> L = nv / 4f = n*346m/s / 4*256Hz = n*0.38 m
</span> <span>Since the only option in your list that is an integer multiple of 0.38 m is 1.35 m
</span><span> I'd say that we're hearing the fourth harmonic.
answer is
</span><span>A. 1.35 m</span><span>
</span>