$\begin {array} {1 1} (a)\;Inside\: the \: pipe\; is \: non-zero\: constant & \quad (b)\;Inside\: the \: pipe\; is \: not\: constant \\ (c)\;Is \: zero\: only\: on \: the \: axis \: of \: the \: pipe & \quad (d)\;Inside \: the \: pipe\: is\: zero \end {array}$
