Let $W$ denote the words in the English dictionary. Define the relation $R$ by

$R=\{(x, y) \in W \times W \mid$ the words $x$ and $y$ have at least one letter in common}. Then, $R$ is

A solid which is not transparent to visible light and whose conductivity increases with temperature is formed by

The circuit has two oppositively connected ideal diodes in parallel. What is the current following in the circuit?

The anode voltage of a photocell is kept fixed. The wavelength $$\lambda $$ of the light falling on the cathode is gradually changed. The plate current $${\rm I}$$ of the photocell varies as follows