Which of the following expressions corresponds to simple harmonic motion along a straight line,where $x$ is the displacement and $a, b, c$ are positive constants?

There exists a close association between the alga and the fungus within a lichen. The fungus

Two capacitors $C_1$ and $C_2$ in a circuit are joined as shown below. The potential of point $A$ is $V_1$ and that of $B$ is $V_2$. The potential of point $D$ will be

$A$ pair of perpendicular straight lines passes through the origin and also through the point of intersection of the curve $x^2+y^2=4$ with $x+y=a$. The set containing the value of $a$ is

For $k=1, 2, 3$,the box $B_k$ contains $k$ red balls and $(k+1)$ white balls. Let $P(B_1)=\frac{1}{2}$,$P(B_2)=\frac{1}{3}$,and $P(B_3)=\frac{1}{6}$. $A$ box is selected at random and a ball is drawn from it. If a red ball is drawn,then the probability that it has come from box $B_2$ is:

$A$ complex of platinum,ammonia,and chloride produces four ions per molecule in the solution. The structure consistent with this observation is: