Which of the following statements is true regarding stationary waves?

The correct statement about stationary waves is that

In a standing wave on a string:

$A$ stretched string is vibrating in its $5^{th}$ harmonic as shown. Consider a particle $1$ (figure). At an instant,this particle is at its mean position and is moving towards its negative extreme. Which of the following sets of particles are in the same phase as particle $1$?

$A$ progressive wave travelling in the positive $x$-direction given by $y=a \cos (k x-\omega t)$ meets a denser surface at $x=0, t=0$. The reflected wave is then given by

$A$ string fixed at both ends vibrates in a resonant mode with a separation of $2.0 \, cm$ between consecutive nodes. For the next higher resonant frequency,this separation is reduced to $1.6 \, cm$. The length of the string is .... $cm$.