Each of the following observations is made when a substance is gently heated in an open test tube.
W. Green CuCO₃·Cu(OH)₂ turns black and releases a colorless gas that turns limewater cloudy.
X. Yellow sulfur powder melts to a red liquid and solidifies back to yellow on cooling.
Y. White AgNO₃ crystals darken to black while a reddish–brown gas is released.
Z. White wax pellets melt to a clear liquid and resolidify unchanged on cooling.
Which ranking lists the observations from strongest to weakest evidence of a chemical change?
a) Y>W>X>Z
b) W>X>Y>Z
c) W>Y>X>Z
d) Y>X>W>Z