Classify the following linear differential equations according $to$ whether they are

time$-$variable $or$ time$-$invariant. Indicate any time$-$variable terms. $1pt.$ each

$(a)\frac{d^{2}y}{d{t}^2}+2y=0$

$(c)(\frac{1}{t+1})\frac{d^{2}y}{d{t}^2}+(\frac{1}{t+1})y=0$

$(b)\frac{d}{dt}(t^{2}y)=0$

$(d)\frac{d^{2}y}{d{t}^2}+(cost)y=0$