From 3 to 7 February 2020, hospital staff (members of the Hospital Authority Employees Alliance) launched a labour strike with the same goal. On 3 February, Chief Executive Carrie Lam said that while only three of the 14 crossing points with mainland China, including the airport, would be left open, she rejected a full border closure because of the "very close relationship" between the people on both sides which entailed "very legitimate and genuine cross-border traffic". People responded negatively to the government's attempt to set up quarantine and clinical centres in neighbourhoods close to residents and marched to express their discontent or blocked roads to thwart the government's plans across the territory. Between late January and early February, improvised explosive devices were found in several locations, and petrol bombs were thrown at four police stations and a patrol car, in a wave of action over the government's failure to close the city's border and supply protective gear. Some of the protest activities have also switched to using an online format. As the coronavirus crisis escalated further in February and March 2020, the scale of the protests dwindled further. Protests activities continued regularly in Tseung Kwan O, Yuen Long and Mong Kok every month. On 19 April 2020, police arrested 15 pro-democracy activists including Jimmy Lai, Martin Lee and Margaret Ng for their activities in 2019, drawing international condemnation. Police have used coronavirus laws banning groups of more than four, for example, to disperse protesters outside Prince Edward station on 31 March, and several shopping mall singing protests as the pandemic relieved in Hong Kong. Hundreds of protesters gathered in shopping malls across the city on Mother's Day on 10 May as riot police quashed plans to hold a pro-democracy march before it began. On 15 May, a 21-year-old young man surnamed Sin was sentenced to 4 years in prison for his participation in the 12 June protest, becoming the first person to be jailed for the charge of rioting since the protest movement started. In October 2019, the pro-democratic bloc in the LegCo launched a filibuster campaign led by vice-chairman Dennis Kwok, who presided over the LegCo meetings, to stall the House Committee election procedures and thereby the second reading of the National Anthem Bill, a bill that was considered to be "evil" by the democrats. A Hong Kong Watch article opined that the filibustering was the only strategy available to the democrats, because of their marginalisation in the legislative council despite them representing the majority of the population. The directors of the Chinese Hong Kong and Macau Affairs Office (HKMAO) and the Liaison Office threatened to disqualify Kwok.