Which of the following is true about congregational prayer?

Ahmad comes late to perform Zuhur prayer at the mosque, at the moment, the imam is in the second rakaat, at the end of the prayer Ahmad gives salam with Imam. 

Ahmad's prayer is valid. 

choose the correct statement about the following hadith

صلاة الجماعة أفضلُ مِن صلاة الفذِّ بسبعٍ وعشرين درجةً

among the following is NOT a requirement to be an imam

the law of congregational prayer for a group of community members

Nadhirah be an Imam to her small son.

the congregational prayer is invalid if there is a kid in the same line (saf)

choose the correct statement about the wisdom of congregational prayer

the distance between the last row inside the mosque and the first row outside the mosque must not exceed ________