dead thread
\\ \overrightarrow{BA} = -3i + 4j + 3k,\ \overrightarrow{BC} = i + 3j - 4k
\overrightarrow{BA} \cdot \overrightarrow{BC} = -3
$As the dot product is negative the vectors make an obtuse angle.$
The question didn't specify that the cup was made purely from one material for q13. It just asked for the one that most likely to be used in the manufacture of the cup.